fix opacity not interpolated in animated brush

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

@@ -12,6 +12,8 @@ namespace Avalonia.Animation.Animators
     /// </summary>
     public class ISolidColorBrushAnimator : Animator<ISolidColorBrush?>
     {
+        private static readonly DoubleAnimator s_doubleAnimator = new DoubleAnimator();
+
         public override ISolidColorBrush? Interpolate(double progress, ISolidColorBrush? oldValue, ISolidColorBrush? newValue)
         {
             if (oldValue is null || newValue is null)
@@ -19,7 +21,9 @@ namespace Avalonia.Animation.Animators
                 return progress >= 0.5 ? newValue : oldValue;
             }
 
-            return new ImmutableSolidColorBrush(ColorAnimator.InterpolateCore(progress, oldValue.Color, newValue.Color));
+            return new ImmutableSolidColorBrush(
+                ColorAnimator.InterpolateCore(progress, oldValue.Color, newValue.Color),
+                s_doubleAnimator.Interpolate(progress, oldValue.Opacity, newValue.Opacity));
         }
 
         public override IDisposable BindAnimation(Animatable control, IObservable<ISolidColorBrush?> instance)