Add APIs for dynamic display lists, using SkDrawable and SkPictureRecorder::endRecordingAsDrawable.

sky/engine/core/painting/Canvas.idl

Index: sky/engine/core/painting/Canvas.idl
diff --git a/sky/engine/core/painting/Canvas.idl b/sky/engine/core/painting/Canvas.idl
index f3be665a353870ff6a36d99ecef396e2a8e14489..dd4e20d6d7fbfd86f1b7dfdf4de1d5bfbbe82c2b 100644
--- a/sky/engine/core/painting/Canvas.idl
+++ b/sky/engine/core/painting/Canvas.idl
@@ -23,7 +23,6 @@
   void clipPath(Path path);
 
   void drawLine(Point p1, Point p2, Paint paint);
-  void drawPicture(Picture picture);
   void drawPaint(Paint paint);
   void drawRect(Rect rect, Paint paint);
   void drawRRect(RRect rrect, Paint paint);
@@ -32,4 +31,6 @@
   void drawPath(Path path, Paint paint);
   void drawImage(Image image, Point p, Paint paint);
   void drawImageRect(Image image, Rect src, Rect dst, Paint paint);
+  void drawPicture(Picture picture);
+  void drawDrawable(Drawable drawable);
 };