Make all mutable brushes implement IMutableBrush.

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, IImageBrush
+    public class ImageBrush : TileBrush, IImageBrush, IMutableBrush
     {
         /// <summary>
         /// Defines the <see cref="Visual"/> property.
@@ -17,14 +17,14 @@ namespace Avalonia.Media
             AvaloniaProperty.Register<ImageBrush, IBitmap>("Source");
 
         /// <summary>
-        /// Initializes a new instance of the <see cref="VisualBrush"/> class.
+        /// Initializes a new instance of the <see cref="ImageBrush"/> class.
         /// </summary>
         public ImageBrush()
         {
         }
 
         /// <summary>
-        /// Initializes a new instance of the <see cref="VisualBrush"/> class.
+        /// Initializes a new instance of the <see cref="ImageBrush"/> class.
         /// </summary>
         /// <param name="source">The image to draw.</param>
         public ImageBrush(IBitmap source)
@@ -40,5 +40,11 @@ namespace Avalonia.Media
             get { return GetValue(SourceProperty); }
             set { SetValue(SourceProperty, value); }
         }
+
+        /// <inheritdoc/>
+        IBrush IMutableBrush.ToImmutable()
+        {
+            return new Immutable.ImmutableImageBrush(this);
+        }
     }
 }

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, ILinearGradientBrush
+    public sealed class LinearGradientBrush : GradientBrush, ILinearGradientBrush, IMutableBrush
     {
         /// <summary>
         /// Defines the <see cref="StartPoint"/> property.
@@ -41,5 +41,11 @@ namespace Avalonia.Media
             get { return GetValue(EndPointProperty); }
             set { SetValue(EndPointProperty, value); }
         }
+
+        /// <inheritdoc/>
+        IBrush IMutableBrush.ToImmutable()
+        {
+            return new Immutable.ImmutableLinearGradientBrush(this);
+        }
     }
 }

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, IRadialGradientBrush
+    public sealed class RadialGradientBrush : GradientBrush, IRadialGradientBrush, IMutableBrush
     {
         /// <summary>
         /// Defines the <see cref="Center"/> property.
@@ -61,5 +61,11 @@ namespace Avalonia.Media
             get { return GetValue(RadiusProperty); }
             set { SetValue(RadiusProperty, value); }
         }
+
+        /// <inheritdoc/>
+        IBrush IMutableBrush.ToImmutable()
+        {
+            return new Immutable.ImmutableRadialGradientBrush(this);
+        }
     }
 }

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, IVisualBrush
+    public class VisualBrush : TileBrush, IVisualBrush, IMutableBrush
     {
         /// <summary>
         /// Defines the <see cref="Visual"/> property.
@@ -40,5 +40,11 @@ namespace Avalonia.Media
             get { return GetValue(VisualProperty); }
             set { SetValue(VisualProperty, value); }
         }
+
+        /// <inheritdoc/>
+        IBrush IMutableBrush.ToImmutable()
+        {
+            return new Immutable.ImmutableVisualBrush(this);
+        }
     }
 }