xwayland: Use frame callbacks for Present vblank events

Instead of only the fallback timer.

Fixes xorg/xserver#854

v2:
* Drop unused frame_callback member of struct xwl_present_window
  (Olivier Fourdan)

Reviewed-by: Olivier Fourdan <ofourdan@redhat.com>
3 jobs for xwayland-present-vblank-frame_callback in 5 minutes and 45 seconds (queued for 8 seconds)