Adds basic sprite sheet support to sprites

sky/engine/core/painting/Canvas.idl

 // 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.
 
 // TODO(mpcomplete): Figure out a better SkMatrix representation.
 interface Canvas {
   // Height and width are used for culling optimizations and do not necessarily
   // imply that the Canvas is backed by a buffer with any specific bounds.
   readonly attribute float height;
   readonly attribute float width;
@@ skipping 15 matching lines @@
 
   void drawLine(float x0, float y0, float x1, float y1, Paint paint);
   void drawPicture(Picture picture);
   void drawPaint(Paint paint);
   void drawRect(Rect rect, Paint paint);
   void drawRRect(RRect rrect, Paint paint);
   void drawOval(Rect rect, Paint paint);
   void drawCircle(float x, float y, float radius, Paint paint);
   void drawPath(Path path, Paint paint);
   void drawImage(Image image, float x, float y, Paint paint);
+  void drawImageRect(Image image, Rect src, Rect dst, Paint paint);
 };