Add interfaces for all brush types.

Also:
- Change `GradientBrush.GradientStops` to be a readonly list, as changes
to it would have been ignored anyway.
- Fix typo in ImageBrush filename.

src/Avalonia.HtmlRenderer/Adapters/AvaloniaAdapter.cs

@@ -80,7 +80,7 @@ namespace TheArtOfDev.HtmlRenderer.Avalonia.Adapters
             {
                 StartPoint = new RelativePoint(x, y, RelativeUnit.Relative), 
                 EndPoint = new RelativePoint(1 - x, 1 - y, RelativeUnit.Relative),
-                GradientStops =
+                GradientStops = new[]
                 {
                     new GradientStop(startColor, 0),
                     new GradientStop(endColor, 1)

src/Avalonia.Visuals/Avalonia.Visuals.csproj

@@ -69,7 +69,13 @@
     <Compile Include="Media\BrushMappingMode.cs" />
     <Compile Include="Media\Color.cs" />
     <Compile Include="Media\Colors.cs" />
+    <Compile Include="Media\IGradientBrush.cs" />
+    <Compile Include="Media\ILinearGradientBrush.cs" />
+    <Compile Include="Media\Imaging\IImageBrush.cs" />
     <Compile Include="Media\Imaging\WritableBitmap.cs" />
+    <Compile Include="Media\IRadialGradientBrush.cs" />
+    <Compile Include="Media\ITileBrush.cs" />
+    <Compile Include="Media\IVisualBrush.cs" />
     <Compile Include="Media\TextWrapping.cs" />
     <Compile Include="Media\TransformGroup.cs" />
     <Compile Include="Media\DashStyle.cs" />
@@ -133,7 +139,7 @@
     <Compile Include="Media\TextHitTestResult.cs" />
     <Compile Include="Media\Transform.cs" />
     <Compile Include="Media\TileBrush.cs" />
-    <Compile Include="Media\ImageBush.cs" />
+    <Compile Include="Media\ImageBrush.cs" />
     <Compile Include="Media\VisualBrush.cs" />
     <Compile Include="Platform\IPlatformSettings.cs" />
     <Compile Include="RelativePoint.cs" />

src/Avalonia.Visuals/Media/GradientBrush.cs

@@ -1,32 +1,52 @@
 // Copyright (c) The Avalonia Project. All rights reserved.
 // Licensed under the MIT license. See licence.md file in the project root for full license information.
 
+using System;
 using System.Collections.Generic;
 using Avalonia.Metadata;
 
 namespace Avalonia.Media
 {
-    public abstract class GradientBrush : Brush
+    /// <summary>
+    /// Base class for brushes that draw with a gradient.
+    /// </summary>
+    public abstract class GradientBrush : Brush, IGradientBrush
     {
+        /// <summary>
+        /// Defines the <see cref="SpreadMethod"/> property.
+        /// </summary>
         public static readonly StyledProperty<GradientSpreadMethod> SpreadMethodProperty =
             AvaloniaProperty.Register<GradientBrush, GradientSpreadMethod>(nameof(SpreadMethod));
 
-        public static readonly StyledProperty<List<GradientStop>> GradientStopsProperty =
+        /// <summary>
-            AvaloniaProperty.Register<GradientBrush, List<GradientStop>>(nameof(Opacity));
+        /// Defines the <see cref="GradientStops"/> property.
+        /// </summary>
+        public static readonly StyledProperty<IReadOnlyList<GradientStop>> GradientStopsProperty =
+            AvaloniaProperty.Register<GradientBrush, IReadOnlyList<GradientStop>>(nameof(Opacity));
 
+        /// <summary>
+        /// Initializes a new instance of the <see cref="GradientBrush"/> class.
+        /// </summary>
         public GradientBrush()
         {
             this.GradientStops = new List<GradientStop>();
         }
 
+        /// <summary>
+        /// Gets or sets the brush's spread method that defines how to draw a gradient that
+        /// doesn't fill the bounds of the destination control.
+        /// </summary>
         public GradientSpreadMethod SpreadMethod
         {
             get { return GetValue(SpreadMethodProperty); }
             set { SetValue(SpreadMethodProperty, value); }
         }
 
+        /// <summary>
+        /// Gets or sets the brush's gradient stops.
+        /// </summary>
         [Content]
-        public List<GradientStop> GradientStops
+        public IReadOnlyList<GradientStop> GradientStops
         {
             get { return GetValue(GradientStopsProperty); }
             set { SetValue(GradientStopsProperty, value); }

src/Avalonia.Visuals/Media/IGradientBrush.cs

@@ -0,0 +1,21 @@
+using System.Collections.Generic;
+
+namespace Avalonia.Media
+{
+    /// <summary>
+    /// A brush that draws with a gradient.
+    /// </summary>
+    public interface IGradientBrush : IBrush
+    {
+        /// <summary>
+        /// Gets the brush's gradient stops.
+        /// </summary>
+        IReadOnlyList<GradientStop> GradientStops { get; }
+
+        /// <summary>
+        /// Gets the brush's spread method that defines how to draw a gradient that doesn't fill
+        /// the bounds of the destination control.
+        /// </summary>
+        GradientSpreadMethod SpreadMethod { get; }
+    }
+}

src/Avalonia.Visuals/Media/ILinearGradientBrush.cs

@@ -0,0 +1,18 @@
+namespace Avalonia.Media
+{
+    /// <summary>
+    /// A brush that draws with a linear gradient.
+    /// </summary>
+    public interface ILinearGradientBrush : IGradientBrush
+    {
+        /// <summary>
+        /// Gets or sets the start point for the gradient.
+        /// </summary>
+        RelativePoint StartPoint { get; }
+
+        /// <summary>
+        /// Gets or sets the end point for the gradient.
+        /// </summary>
+        RelativePoint EndPoint { get; }
+    }
+}

src/Avalonia.Visuals/Media/IRadialGradientBrush.cs

@@ -0,0 +1,24 @@
+namespace Avalonia.Media
+{
+    /// <summary>
+    /// Paints an area with a radial gradient.
+    /// </summary>
+    public interface IRadialGradientBrush : IGradientBrush
+    {
+        /// <summary>
+        /// Gets the start point for the gradient.
+        /// </summary>
+        RelativePoint Center { get; }
+
+        /// <summary>
+        /// Gets the location of the two-dimensional focal point that defines the beginning of the
+        /// gradient.
+        /// </summary>
+        RelativePoint GradientOrigin { get; }
+
+        /// <summary>
+        /// Gets the horizontal and vertical radius of the outermost circle of the radial gradient.
+        /// </summary>
+        double Radius { get; }
+    }
+}

src/Avalonia.Visuals/Media/ITileBrush.cs

@@ -0,0 +1,39 @@
+namespace Avalonia.Media
+{
+    /// <summary>
+    /// A brush which displays a repeating image.
+    /// </summary>
+    public interface ITileBrush : IBrush
+    {
+        /// <summary>
+        /// Gets the horizontal alignment of a tile in the destination.
+        /// </summary>
+        AlignmentX AlignmentX { get; }
+
+        /// <summary>
+        /// Gets the horizontal alignment of a tile in the destination.
+        /// </summary>
+        AlignmentY AlignmentY { get; }
+
+        /// <summary>
+        /// Gets the rectangle on the destination in which to paint a tile.
+        /// </summary>
+        RelativeRect DestinationRect { get; }
+
+        /// <summary>
+        /// Gets the rectangle of the source image that will be displayed.
+        /// </summary>
+        RelativeRect SourceRect { get; }
+
+        /// <summary>
+        /// Gets a value indicating how the source rectangle will be stretched to fill the
+        /// destination rect.
+        /// </summary>
+        Stretch Stretch { get; }
+
+        /// <summary>
+        /// Gets the brush's tile mode.
+        /// </summary>
+        TileMode TileMode { get; }
+    }
+}

src/Avalonia.Visuals/Media/IVisualBrush.cs

@@ -0,0 +1,15 @@
+using Avalonia.VisualTree;
+
+namespace Avalonia.Media
+{
+    /// <summary>
+    /// Paints an area with an <see cref="IVisual"/>.
+    /// </summary>
+    public interface IVisualBrush : ITileBrush
+    {
+        /// <summary>
+        /// Gets the visual to draw.
+        /// </summary>
+        IVisual Visual { get; }
+    }
+}

src/Avalonia.Visuals/Media/ImageBush.cs → src/Avalonia.Visuals/Media/ImageBrush.cs

@@ -8,7 +8,7 @@ namespace Avalonia.Media
     /// <summary>
     /// Paints an area with an <see cref="IBitmap"/>.
     /// </summary>
-    public class ImageBrush : TileBrush
+    public class ImageBrush : TileBrush, IImageBrush
     {
         /// <summary>
         /// Defines the <see cref="Visual"/> property.

src/Avalonia.Visuals/Media/Imaging/IImageBrush.cs

@@ -0,0 +1,15 @@
+using Avalonia.Media.Imaging;
+
+namespace Avalonia.Media
+{
+    /// <summary>
+    /// Paints an area with an <see cref="IBitmap"/>.
+    /// </summary>
+    public interface IImageBrush : ITileBrush
+    {
+        /// <summary>
+        /// Gets the image to draw.
+        /// </summary>
+        IBitmap Source { get; }
+    }
+}

src/Avalonia.Visuals/Media/LinearGradientBrush.cs

@@ -6,7 +6,7 @@ namespace Avalonia.Media
     /// <summary>
     /// A brush that draws with a linear gradient.
     /// </summary>
-    public sealed class LinearGradientBrush : GradientBrush
+    public sealed class LinearGradientBrush : GradientBrush, ILinearGradientBrush
     {
         /// <summary>
         /// Defines the <see cref="StartPoint"/> property.

src/Avalonia.Visuals/Media/RadialGradientBrush.cs

@@ -7,7 +7,7 @@ namespace Avalonia.Media
     /// Paints an area with a radial gradient. A focal point defines the beginning of the gradient, 
     /// and a circle defines the end point of the gradient.
     /// </summary>
-    public sealed class RadialGradientBrush : GradientBrush
+    public sealed class RadialGradientBrush : GradientBrush, IRadialGradientBrush
     {
         /// <summary>
         /// Defines the <see cref="Center"/> property.
@@ -43,7 +43,8 @@ namespace Avalonia.Media
         }
 
         /// <summary>
-        /// Gets or sets the location of the two-dimensional focal point that defines the beginning of the gradient.
+        /// Gets or sets the location of the two-dimensional focal point that defines the beginning
+        /// of the gradient.
         /// </summary>
         public RelativePoint GradientOrigin
         {
@@ -52,7 +53,8 @@ namespace Avalonia.Media
         }
 
         /// <summary>
-        /// Gets or sets the horizontal and vertical radius of the outermost circle of the radial gradient.
+        /// Gets or sets the horizontal and vertical radius of the outermost circle of the radial
+        /// gradient.
         /// </summary>
         public double Radius
         {

src/Avalonia.Visuals/Media/TileBrush.cs

@@ -37,7 +37,7 @@ namespace Avalonia.Media
     /// <summary>
     /// Base class for brushes which display repeating images.
     /// </summary>
-    public abstract class TileBrush : Brush
+    public abstract class TileBrush : Brush, ITileBrush
     {
         /// <summary>
         /// Defines the <see cref="AlignmentX"/> property.

src/Avalonia.Visuals/Media/VisualBrush.cs

@@ -8,7 +8,7 @@ namespace Avalonia.Media
     /// <summary>
     /// Paints an area with an <see cref="IVisual"/>.
     /// </summary>
-    public class VisualBrush : TileBrush
+    public class VisualBrush : TileBrush, IVisualBrush
     {
         /// <summary>
         /// Defines the <see cref="Visual"/> property.

tests/Avalonia.RenderTests/Media/LinearGradientBrushTests.cs

@@ -42,7 +42,7 @@ namespace Avalonia.Direct2D1.RenderTests.Media
                     {
                         StartPoint = new RelativePoint(0, 0.5, RelativeUnit.Relative),
                         EndPoint = new RelativePoint(1, 0.5, RelativeUnit.Relative),
-                        GradientStops =
+                        GradientStops = new[]
                         {
                             new GradientStop { Color = Colors.Red, Offset = 0 },
                             new GradientStop { Color = Colors.Blue, Offset = 1 }
@@ -73,7 +73,7 @@ namespace Avalonia.Direct2D1.RenderTests.Media
                     {
                         StartPoint = new RelativePoint(0.5, 0, RelativeUnit.Relative),
                         EndPoint = new RelativePoint(0.5, 1, RelativeUnit.Relative),
-                        GradientStops =
+                        GradientStops = new[]
                         {
                             new GradientStop { Color = Colors.Red, Offset = 0 },
                             new GradientStop { Color = Colors.Blue, Offset = 1 }