Store the alarms in a different hashtable, and look up the MetaSyncCounter
right away. It so far avoids the MetaWindow middle man, but will also be
simpler when each window can possibly have more than one active alarms.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2175>
This helper struct takes care of the handling of requests and alarms
in order to satisfy NET_WM_SYNC_REQUEST. It will be necessary to
decouple rendering of windows and frames in future commits, so each
window may need its own synchronization and accounting.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2175>