Add Canvas.drawPicture

sky/examples/raw/draw_picture_into_canvas.sky

 <script>
 import "dart:sky";
 
 void main() {
   var root = document.createElement('div');
   root.style['display'] = 'paragraph';
   root.appendChild(new Text('Hello World'));
   document.appendChild(root);
   root.offsetWidth; // force layout.
 
   double width = window.innerWidth.toDouble();
   double height = window.innerHeight.toDouble();
+  PictureRecorder stampRecorder = new PictureRecorder(width, height);
+  root.paint(stampRecorder);
+  Picture stamp = stampRecorder.endRecording();
+
   PictureRecorder recorder = new PictureRecorder(width, height);
   Paint paint = new Paint()..setARGB(255, 0, 255, 0);
   recorder.drawCircle(50.0, 50.0, 50.0, paint);
   recorder.translate(10.0, 10.0);
-  root.paint(recorder);
+  recorder.drawPicture(stamp);
   recorder.translate(10.0, 10.0);
-  root.paint(recorder);
+  recorder.drawPicture(stamp);
   recorder.translate(10.0, 10.0);
-  root.paint(recorder);
+  recorder.drawPicture(stamp);
   document.rootPicture = recorder.endRecording();
 }
 </script>