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/Drawable.idl b/sky/engine/core/painting/PaintingNode.idl
similarity index 68%
copy from sky/engine/core/painting/Drawable.idl
copy to sky/engine/core/painting/PaintingNode.idl
index a02c65503c87e0528eef97abb3eb9a338ff6f81f..520d8fd423afa1fbbeabad828ed6e0a7dbdb86ac 100644
--- a/sky/engine/core/painting/Drawable.idl
+++ b/sky/engine/core/painting/PaintingNode.idl
@@ -1,6 +1,9 @@
 // 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.
-interface Drawable {
+[
+  Constructor(),
+] interface PaintingNode {
+  void setBackingDrawable(Drawable drawable);
   Picture newPictureSnapshot();
 };