Correctly snap dirty rects to device pixels.

src/Avalonia.Visuals/Rendering/DeferredRenderer.cs

@@ -443,11 +443,12 @@ namespace Avalonia.Rendering
         private static Rect SnapToDevicePixels(Rect rect, double scale)
         {
             return new Rect(
-                Math.Floor(rect.X * scale) / scale,
+                new Point(
-                Math.Floor(rect.Y * scale) / scale,
+                    Math.Floor(rect.X * scale) / scale,
-                Math.Ceiling(rect.Width * scale) / scale,
+                    Math.Floor(rect.Y * scale) / scale),
-                Math.Ceiling(rect.Height * scale) / scale);
+                new Point(
-                
+                    Math.Ceiling(rect.Right * scale) / scale,
+                    Math.Ceiling(rect.Bottom * scale) / scale));
         }
 
         private void RenderOverlay(Scene scene, ref IDrawingContextImpl parentContent)