First pass layer integration

Would be nice if we had a better way to handle the move forward, back,
send to front, send to back stuff.
Hacked in a UI to show layers.

Need to come up with a solution to trigger a re-draw as layer visibility
changes.
Removed extra using.
Pending render

Only issue a single invalidate for each request to redraw the canvas.
Redraw when scene changes.

This is getting to be really clunky.