Adds basic sprite sheet support to sprites

sky/engine/core/painting/Canvas.cpp

Index: sky/engine/core/painting/Canvas.cpp
diff --git a/sky/engine/core/painting/Canvas.cpp b/sky/engine/core/painting/Canvas.cpp
index cc9a9e5bbf193690d7a6c440123b9a69d7bcb86d..37644c0c60460e90ac5b1884a89f89c9f818458d 100644
--- a/sky/engine/core/painting/Canvas.cpp
+++ b/sky/engine/core/painting/Canvas.cpp
@@ -218,6 +218,14 @@ void Canvas::drawImage(const CanvasImage* image,
   m_canvas->drawBitmap(image->bitmap(), x, y, &paint->paint());
 }
 
+void Canvas::drawImageRect(const CanvasImage* image, Rect& src, Rect& dst, Paint* paint) {
+  if (!m_canvas)
+    return;
+  ASSERT(image);

[Hixie, 2015/06/23 22:03:52]

assert that paint is not null (also should happen in drawImage()).

+  ASSERT(m_displayList->isRecording());
+  m_canvas->drawBitmapRectToRect(image->bitmap(), &src.sk_rect, dst.sk_rect, &paint->paint());
+}
+
 PassRefPtr<DisplayList> Canvas::finishRecording()
 {
     if (!isRecording())