Introduce CanvasFrontend

Add a front-end to convert between "stateful" canvas
and "semi-statelss" CanvasOps. Handles tracking
save/restores, clips, and transforms, and injects
the transform on all draw ops.

Test: CanvasFrontend unit tests

Change-Id: Ifbcd14600690f10717047c50954ab54b4fc27aee