Fixed Skia framebuffer bitmap test

tests/Avalonia.RenderTests/Media/BitmapTests.cs

@@ -81,6 +81,7 @@ namespace Avalonia.Direct2D1.RenderTests.Media
             using (var target = r.CreateRenderTarget(new object[] { fb }))
             using (var ctx = target.CreateDrawingContext(null))
             {
+                ctx.Clear(Colors.Transparent);
                 ctx.PushOpacity(0.8);
                 ctx.FillRectangle(Brushes.Chartreuse, new Rect(0, 0, 20, 100));
                 ctx.FillRectangle(Brushes.Crimson, new Rect(20, 0, 20, 100));