Use readonly structs where possible.

C# 7.2 introduced [readonly structs](https://docs.microsoft.com/en-us/dotnet/csharp/reference-semantics-with-value-types). Using this construct we can make sure that future modifications to these structs don't make them immutable and C# can generate more efficient code.

src/Avalonia.Visuals/Avalonia.Visuals.csproj

@@ -3,6 +3,7 @@
     <TargetFramework>netstandard2.0</TargetFramework>
     <GenerateAssemblyInfo>false</GenerateAssemblyInfo>
     <RootNamespace>Avalonia</RootNamespace>
+    <LangVersion>latest</LangVersion>
   </PropertyGroup>
   <PropertyGroup Condition=" '$(Configuration)' == 'Debug' ">
     <DebugSymbols>true</DebugSymbols>

src/Avalonia.Visuals/Matrix.cs

@@ -10,7 +10,7 @@ namespace Avalonia
     /// <summary>
     /// A 2x3 matrix.
     /// </summary>
-    public struct Matrix
+    public readonly struct Matrix
     {
         private readonly double _m11;
         private readonly double _m12;

src/Avalonia.Visuals/Media/Color.cs

@@ -11,7 +11,7 @@ namespace Avalonia.Media
     /// <summary>
     /// An ARGB color.
     /// </summary>
-    public struct Color
+    public readonly struct Color
     {
         /// <summary>
         /// Gets or sets the Alpha component of the color.

src/Avalonia.Visuals/Media/DrawingContext.cs

@@ -19,7 +19,7 @@ namespace Avalonia.Media
             ? new Stack<TransformContainer>()
             : TransformStackPool.Pop();
 
-        struct TransformContainer
+        readonly struct TransformContainer
         {
             public readonly Matrix LocalTransform;
             public readonly Matrix ContainerTransform;
@@ -147,7 +147,7 @@ namespace Avalonia.Media
             }
         }
 
-        public struct PushedState : IDisposable
+        public readonly struct PushedState : IDisposable
         {
             private readonly int _level;
             private readonly DrawingContext _context;

src/Avalonia.Visuals/Media/Immutable/ImmutableSolidColorBrush.cs

@@ -6,7 +6,7 @@ namespace Avalonia.Media.Immutable
     /// <summary>
     /// Fills an area with a solid color.
     /// </summary>
-    public struct ImmutableSolidColorBrush : ISolidColorBrush
+    public readonly struct ImmutableSolidColorBrush : ISolidColorBrush
     {
         /// <summary>
         /// Initializes a new instance of the <see cref="ImmutableSolidColorBrush"/> class.

src/Avalonia.Visuals/Point.cs

@@ -10,7 +10,7 @@ namespace Avalonia
     /// <summary>
     /// Defines a point.
     /// </summary>
-    public struct Point
+    public readonly struct Point
     {
         /// <summary>
         /// The X position.

src/Avalonia.Visuals/Rect.cs

@@ -10,7 +10,7 @@ namespace Avalonia
     /// <summary>
     /// Defines a rectangle.
     /// </summary>
-    public struct Rect
+    public readonly struct Rect
     {
         /// <summary>
         /// An empty rectangle.

src/Avalonia.Visuals/RelativePoint.cs

@@ -27,7 +27,7 @@ namespace Avalonia
     /// <summary>
     /// Defines a point that may be defined relative to a containing element.
     /// </summary>
-    public struct RelativePoint : IEquatable<RelativePoint>
+    public readonly struct RelativePoint : IEquatable<RelativePoint>
     {
         /// <summary>
         /// A point at the top left of the containing element.
@@ -44,7 +44,7 @@ namespace Avalonia
         /// </summary>
         public static readonly RelativePoint BottomRight = new RelativePoint(1, 1, RelativeUnit.Relative);
 
-        private Point _point;
+        private readonly Point _point;
 
         private readonly RelativeUnit _unit;
 

src/Avalonia.Visuals/RelativeRect.cs

@@ -10,7 +10,7 @@ namespace Avalonia
     /// <summary>
     /// Defines a rectangle that may be defined relative to a containing element.
     /// </summary>
-    public struct RelativeRect : IEquatable<RelativeRect>
+    public readonly struct RelativeRect : IEquatable<RelativeRect>
     {
         /// <summary>
         /// A rectangle that represents 100% of an area.

src/Avalonia.Visuals/Rendering/SceneGraph/DeferredDrawingContextImpl.cs

@@ -319,7 +319,7 @@ namespace Avalonia.Rendering.SceneGraph
             }
         }
 
-        public struct UpdateState : IDisposable
+        public readonly struct UpdateState : IDisposable
         {
             public UpdateState(
                 DeferredDrawingContextImpl owner,

src/Avalonia.Visuals/Size.cs

@@ -10,7 +10,7 @@ namespace Avalonia
     /// <summary>
     /// Defines a size.
     /// </summary>
-    public struct Size
+    public readonly struct Size
     {
         /// <summary>
         /// A size representing infinity.

src/Avalonia.Visuals/Thickness.cs

@@ -10,7 +10,7 @@ namespace Avalonia
     /// <summary>
     /// Describes the thickness of a frame around a rectangle.
     /// </summary>
-    public struct Thickness
+    public readonly struct Thickness
     {
         /// <summary>
         /// The thickness on the left.

src/Avalonia.Visuals/Vector.cs

@@ -10,7 +10,7 @@ namespace Avalonia
     /// <summary>
     /// Defines a vector.
     /// </summary>
-    public struct Vector
+    public readonly struct Vector
     {
         /// <summary>
         /// The X vector.

src/Avalonia.Visuals/VisualTree/TransformedBounds.cs

@@ -8,7 +8,7 @@ namespace Avalonia.VisualTree
     /// <summary>
     /// Holds information about the bounds of a control, together with a transform and a clip.
     /// </summary>
-    public struct TransformedBounds
+    public readonly struct TransformedBounds
     {
         /// <summary>
         /// Initializes a new instance of the <see cref="TransformedBounds"/> struct.

src/Shared/RenderHelpers/ArcToHelper.cs

@@ -982,7 +982,7 @@ namespace Avalonia.RenderHelpers
             /// At some point I did not trust the WPF Matrix struct, and wrote my own simple one -_-
             /// This is supposed to be replaced with proper WPF Matrices everywhere
             /// </summary>
-            private struct SimpleMatrix
+            private readonly struct SimpleMatrix
             {
                 private readonly double _a, _b, _c, _d;
 

src/Skia/Avalonia.Skia/Avalonia.Skia.csproj

@@ -5,7 +5,8 @@
     <RootNamespace>Avalonia.Skia</RootNamespace>
     <AssemblyName>Avalonia.Skia</AssemblyName>
     <IncludeLinuxSkia>true</IncludeLinuxSkia>
-	<AllowUnsafeBlocks>true</AllowUnsafeBlocks>
+    <LangVersion>latest</LangVersion>
+    <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
   </PropertyGroup>
   <ItemGroup>
     <Compile Include="..\..\Shared\SharedAssemblyInfo.cs">

src/Windows/Avalonia.Direct2D1/Avalonia.Direct2D1.csproj

@@ -2,6 +2,7 @@
   <PropertyGroup>
     <TargetFramework>netstandard2.0</TargetFramework>
     <GenerateAssemblyInfo>false</GenerateAssemblyInfo>
+    <LangVersion>latest</LangVersion>
   </PropertyGroup>
   <ItemGroup>
     <Compile Include="..\..\Shared\SharedAssemblyInfo.cs">

src/Windows/Avalonia.Direct2D1/OptionalDispose.cs

@@ -2,7 +2,7 @@
 
 namespace Avalonia.Direct2D1
 {
-    public struct OptionalDispose<T> : IDisposable where T : IDisposable
+    public readonly struct OptionalDispose<T> : IDisposable where T : IDisposable
     {
         private readonly bool _dispose;