compositor-x11: Adjust to event loop changes

compositor-x11: Adjust to event loop changes

Instead of having a separate check function, we just mark our x11 event
source as needing a post-dispatch check.  The event loop will call our
dispatch function again after all dispatching is done, this time with
mask = 0.  If we don't process any events, return 0, so the event loop
doesn't keep calling us.