Merge pull request #1697 from AvaloniaUI/ghosting-fix

Fix ghosting artifacts

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

@@ -54,6 +54,8 @@ namespace Avalonia.Rendering
             {
                 using (var context = new DrawingContext(_renderTarget.CreateDrawingContext(this)))
                 {
+                    context.PlatformImpl.Clear(Colors.Transparent);
+
                     using (context.PushTransformContainer())
                     {
                         Render(context, _root, _root.Bounds);

src/Windows/Avalonia.Direct2D1/SwapChainRenderTarget.cs

@@ -118,7 +118,7 @@ namespace Avalonia.Direct2D1
                     new BitmapProperties1(
                         new PixelFormat
                         {
-                            AlphaMode = AlphaMode.Ignore,
+                            AlphaMode = AlphaMode.Premultiplied,
                             Format = Format.B8G8R8A8_UNorm
                         },
                         _savedDpi.Width,