Let userland retain the window backing store while drawing into it.

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.

Kernel/GUITypes.h

@@ -34,6 +34,7 @@ struct GUI_WindowParameters {
 };
 
 struct GUI_WindowBackingStoreInfo {
+    void* backing_store_id;
     GUI_Size size;
     size_t bpp;
     size_t pitch;