used by Mutter to compute which windows should be mapped
used by Mutter to see if there's a fullscreen window
call at a very low priority (can be blocked by running animations or redrawing applications)
call in a resize processing phase that is done before GTK+ repainting (including window borders) is done.
used by Mutter to send it's idea of the stacking order to the server
call before the stage is redrawn