Fix ghosting artifacts on both Renderers #1687

src/Avalonia.Visuals/Rendering/DeferredRenderer.cs

@@ -337,6 +337,8 @@ namespace Avalonia.Rendering
 
         private void RenderComposite(Scene scene, IDrawingContextImpl context)
         {
+            context.Clear(Colors.Transparent);
+            
             var clientRect = new Rect(scene.Size);
 
             foreach (var layer in scene.Layers)

src/Avalonia.Visuals/Rendering/ImmediateRenderer.cs

@@ -224,6 +224,9 @@ namespace Avalonia.Rendering
             var clipToBounds = visual.ClipToBounds;
             var bounds = new Rect(visual.Bounds.Size);
 
+            if (visual == _root)
+                context.PlatformImpl.Clear(Colors.Transparent);
+
             if (visual.IsVisible && opacity > 0)
             {
                 var m = Matrix.CreateTranslation(visual.Bounds.Position);