xwayland: Complete "synchronous" Present flips from xwl_present_msc_bump

Completing them from xwl_present_sync_callback had at least two issues:

* It was before the MSC was incremented in xwl_present_frame_callback,
  so the MSC value in the completion event could be lower than the
  target specified by the client. This could cause hangs with the Mesa
  Vulkan drivers.
* It allowed clients to run at a frame-rate higher than the Wayland
  compositor's frame-rate, wasting energy on generating frames which
  were never displayed. This isn't expected to happen unless the client
  specified PresentOptionAsync (in which case flips are still completed
  from xwl_present_sync_callback, allowing higher frame-rates).

* Make xwl_present_has_events return true when there's a pending
  "synchronous" flip, so those complete after at most ~1 second even if
  the Wayland server doesn't send a frame event.

Bugzilla: https://bugs.freedesktop.org/106713
1 job for xwl_present_msc_bump-sync_flip in 2 minutes and 52 seconds (queued for 2 minutes and 35 seconds)