Clear the intermediate render target.

This prevents the glitching we've been seeing in DropDown with a control
item. For some reason doing a FillRectange on it didn't work. Closes
#108.

src/Shared/RenderHelpers/TileBrushImplHelper.cs

@@ -105,7 +105,6 @@ namespace Perspex.RenderHelpers
                 }
                 else if (_visualBrush != null)
                 {
-                    ctx.FillRectangle(Brushes.Black, new Rect(new Point(0, 0), IntermediateSize));
                     ctx.Render(_visualBrush.Visual);
                 }
             }

src/Windows/Perspex.Direct2D1/Media/TileBrushImpl.cs

@@ -25,7 +25,10 @@ namespace Perspex.Direct2D1.Media
             using (var intermediate = new BitmapRenderTarget(target, CompatibleRenderTargetOptions.None, helper.IntermediateSize.ToSharpDX()))
             {
                 using (var ctx = new RenderTarget(intermediate).CreateDrawingContext())
+                {
+                    intermediate.Clear(null);
                     helper.DrawIntermediate(ctx);
+                }
 
                 PlatformBrush = new BitmapBrush(
                     target,