xwayland: Always hook up frame_callback_list in xwl_present_queue_vblank

Even if there's no pending frame callback yet.

Without this, if there was no pending frame callback yet in
xwl_present_queue_vblank, xwl_present_msc_bump would only get called
from xwl_present_timer_callback, resulting in the MSC ticking at ~58
Hertz.

Doing this requires some adjustments elsewhere:

1. xwl_present_reset_timer needs to check for a pending frame callback
   as well.
2. xwl_window_create_frame_callback needs to call
   xwl_present_reset_timer for all child windows hooked up to
   frame_callback_list, to make sure the timer length takes the pending
   frame callback into account.
3. xwl_present_flip needs to hook up the window to frame_callback_list
   before calling xwl_window_create_frame_callback, for 2. to work.

Closes: xorg/xserver#1309
Fixes: 9b31358c ("xwayland: Use frame callbacks for Present vblank events")
Reviewed-by: Olivier Fourdan <ofourdan@redhat.com>
(cherry picked from commit 9e5a3796)