Add `Rect` animation handler.

src/Avalonia.Visuals/Animation/Animators/RectAnimator.cs

@@ -0,0 +1,23 @@
+using System;
+using Avalonia.Logging;
+using Avalonia.Media;
+
+namespace Avalonia.Animation.Animators
+{
+    /// <summary>
+    /// Animator that handles <see cref="Rect"/> properties.
+    /// </summary>
+    public class RectAnimator : Animator<Rect>
+    {
+        public override Rect Interpolate(double progress, Rect oldValue, Rect newValue)
+        {
+            var deltaPos = newValue.Position - oldValue.Position;
+            var deltaSize = newValue.Size - oldValue.Size;
+
+            var newPos = (deltaPos * progress) + oldValue.Position;
+            var newSize = (deltaSize * progress) + oldValue.Size;
+
+            return new Rect(newPos, newSize);
+        }
+    }
+}

src/Avalonia.Visuals/Media/Color.cs

@@ -15,9 +15,9 @@ namespace Avalonia.Media
     {
         static Color()
         {
-            Animation.Animation.RegisterAnimator<ColorAnimator>(prop => typeof(Color).IsAssignableFrom(prop.PropertyType));
+            Animation.Animation.RegisterAnimator<ColorAnimator>(prop => typeof(IBrush).IsAssignableFrom(prop.PropertyType));
         }
-
+        
         /// <summary>
         /// Gets or sets the Alpha component of the color.
         /// </summary>

src/Avalonia.Visuals/Rect.cs

@@ -3,6 +3,7 @@
 
 using System;
 using System.Globalization;
+using Avalonia.Animation.Animators;
 using Avalonia.Utilities;
 
 namespace Avalonia
@@ -12,6 +13,11 @@ namespace Avalonia
     /// </summary>
     public readonly struct Rect
     {
+        static Rect()
+        {
+            Animation.Animation.RegisterAnimator<RectAnimator>(prop => typeof(Rect).IsAssignableFrom(prop.PropertyType));
+        }
+
         /// <summary>
         /// An empty rectangle.
         /// </summary>