wayland: Emit frame events in GSource after "empty" updates

Under certain conditions a stage-view update does not trigger a kms
update.

In such cases we still want the next update to run within the same
refresh cycle, as otherwise we'd waste the remaining time in the
current one.
At the same time we currently use the `after-update` signal for Wayland
frame events, which again may result in more "empty" updates -
creating an unthrottled feedback loop. This can trigger excessive
load both in the compositor as well as in clients.

Introduce a new GSource that is dispatched once per refresh cycle at
maximum per stage view and use it to emit frame events. Do so by
computing the time from when on we can be sure that an update resulting
from a client commit would certainly get scheduled to the next refresh
cycle.

Note: this only works on the native backend. Given that chances are
small that we hit the corresponding issue on e.g. the nested backend,
stick to the previous behavior there for now.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2823>