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).
v2:
* 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
ace551d8a2