present: Leave vblank on window list until flip complete

present: Leave vblank on window list until flip complete

If the window is destroyed, then we've got cleanup work to do, even if
the vblank has already been executed -- we need to clear the window
pointer so that we don't try to deliver events to it.

Leaving it on the window list meant that when walking that list, we
need to know whether the vblank is waiting to be executed or waiting
for the flip to complete, so a new 'queued' flag was added to the
vblank to distinguish between the two states.

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