Decouple Canvas from DisplayList and map Picture and PictureRecorder more directly to their Skia co…

sky/sdk/lib/rendering/stack.dart

Index: sky/sdk/lib/rendering/stack.dart
diff --git a/sky/sdk/lib/rendering/stack.dart b/sky/sdk/lib/rendering/stack.dart
index ed8377b00e580465a4e43e4f0b472cd29f2ab2ed..72a7cb62665d6fdda0596cc108c47d02d3660ba5 100644
--- a/sky/sdk/lib/rendering/stack.dart
+++ b/sky/sdk/lib/rendering/stack.dart
@@ -193,7 +193,7 @@ class RenderStack extends RenderBox with ContainerRenderObjectMixin<RenderBox, S
     defaultHitTestChildren(result, position: position);
   }
 
-  void paint(RenderObjectDisplayList canvas) {
+  void paint(RenderCanvas canvas) {
     defaultPaint(canvas);
   }
 }