If a second `set_{sync,complete}_pending` was queued before the idle
handler had flushed the first then one of them would be forgotten.
It would stay queued forever and never emitted as a notification.
This could happen repeatedly causing a slow leak. But worse still,
`clutter-stage-cogl` would then have `pending_swaps` permanently stuck
above zero preventing the presentation timing logic from being used.
The problem is that a boolean can only count to one, but in some cases
(triple buffering, whether intentional or accidental #334) we need it to
count to two. So just change booleans to integers and count properly.
https://gitlab.gnome.org/GNOME/mutter/merge_requests/216
20c1295a33