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.
Previously we'd blit every pixel in every window that intersected any dirty
rect to the back buffer. With this patch, we limit ourselves to blitting the
pixels inside the actual dirty rects.
There's still a lot of optimizations to make in this code.
