Adds support for transfer mode in Paint and SpriteNode classes

sky/engine/core/painting/Paint.h

Index: sky/engine/core/painting/Paint.h
diff --git a/sky/engine/core/painting/Paint.h b/sky/engine/core/painting/Paint.h
index 402080a1a85220d5dc8b5d233219e328a9ba0b25..71ac40573207b2a3fe8d5fd720c16070fd404c92 100644
--- a/sky/engine/core/painting/Paint.h
+++ b/sky/engine/core/painting/Paint.h
@@ -6,6 +6,7 @@
 #define SKY_ENGINE_CORE_PAINTING_PAINT_H_
 
 #include "sky/engine/core/painting/CanvasColor.h"
+#include "sky/engine/core/painting/TransferMode.h"
 #include "sky/engine/tonic/dart_wrappable.h"
 #include "sky/engine/wtf/PassRefPtr.h"
 #include "sky/engine/wtf/RefCounted.h"
@@ -43,6 +44,8 @@ public:
     const SkPaint& paint() const { return m_paint; }
     void setPaint(const SkPaint& paint) { m_paint = paint; }
 
+    void setTransferMode(SkXfermode::Mode transfer_mode);
+
 private:
     Paint();