Sky: Add a DrawLooper interface to the painting API to be used for shadows.

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 a592623c0a7434076e0d944182bbf6e47dde5a99..d547474f6066fce99563c2da23f840bf7ebd1016 100644
--- a/sky/engine/core/painting/Paint.h
+++ b/sky/engine/core/painting/Paint.h
@@ -12,6 +12,8 @@
 
 namespace blink {
 
+class DrawLooper;
+
 class Paint : public RefCounted<Paint>, public DartWrappable {
     DEFINE_WRAPPERTYPEINFO();
 public:
@@ -31,8 +33,10 @@ public:
     {
         m_paint.setARGB(a, r, g, b);
     }
+    void setDrawLooper(DrawLooper* looper);
 
     const SkPaint& paint() const { return m_paint; }
+    void setPaint(const SkPaint& paint) { m_paint = paint; }
 
 private:
     Paint();