This patch contains Dart bindings for adding a child scene when running in
Mozart. The child scene is currently ignored, but a later patch will actually
incoporate it into the tree.
When rasterzing with a non-zero left and top offset, we need to apply the scale
first so that we're translating in the correct coordinate system.
Fixes https://github.com/flutter/flutter/issues/1292
The matrix argument to drawPicture doesn't seem to do what we want exactly.
Instead, use the normal matrix in the canvas. Also, handle cull rects with
non-zero left and top coordinates.
Fixes#1229
