present: Signal destroyed flip window with vblank->window == NULL

present: Signal destroyed flip window with vblank->window == NULL

This eliminates dereferencing freed window pointers when there is a
flip for that window in progress. The flip will complete, and then
immediately get undone (as we can't stop an in-progress flip).

Remove the vblank->window_destroyed field as we can signal this with
vblank->window == NULL instead.

Change check to vblank->window == NULL in:

	present_flip_notify

Add check for vblank->window == NULL in:
	present_vblank_notify
	present_execute

present_flip_notify was also using vblank->window->drawable.pScreen,
so stop doing that and use vblank->screen instead.

Signed-off-by: Keith Packard <keithp@keithp.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>