To start painting, call:
gui$get_window_backing_store()
Then finish up with:
gui$release_window_backing_store()
Process will retain the underlying GraphicsBitmap behind the scenes.
This fixes racing between the WindowServer and GUI clients.
This patch also adds a WSWindowLocker that is exactly what it sounds like.
There is some trouble here with the asynchronous nature of WindowServer
and the single per-window backing store we're drawing into. If we start
repainting a widget with a pending invalidation, that invalidation might
get flushed by the WindowServer mid-paint.
