Make rendering use PaintingNodes for increased efficiency.

sky/engine/core/painting/PaintingNode.idl

Index: sky/engine/core/painting/PaintingNode.idl
diff --git a/sky/engine/core/painting/RRect.idl b/sky/engine/core/painting/PaintingNode.idl
similarity index 71%
copy from sky/engine/core/painting/RRect.idl
copy to sky/engine/core/painting/PaintingNode.idl
index 16d83c73282ccd0c46b267d65ebd8bff8fa37cb7..f084e14b88756c25d0739bc26561ec66cff45967 100644
--- a/sky/engine/core/painting/RRect.idl
+++ b/sky/engine/core/painting/PaintingNode.idl
@@ -1,9 +1,8 @@
 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
-
 [
   Constructor(),
-] interface RRect {
-  void setRectXY(Rect rect, float xRad, float yRad);
+] interface PaintingNode {
+  void setBackingDrawable(Drawable drawable);
 };