In this scenario:
1. Only an "empty" content update (which results in no visible output
change) from client A arrives before the frame deadline, so a frame
event is sent for that at the deadline.
2. Another "empty" content update from client A results in a frame event
being scheduled for the next frame deadline.
3. A non-"empty" content update from client B arrives before start of
vblank, and the resulting output frame manages to hit the next
display refresh cycle.
The result was that the frame events from steps 1+2 were sent during the
same display refresh cycle, so the frame rate reported by client A was
higher than the display refresh rate.
This change fixes that, at the cost of frame events being sent out
later in the display refresh cycle if there's no new frame for the
next cycle.
Closes: https://gitlab.gnome.org/GNOME/mutter/-/issues/3559
Fixes: 8f27ebf87eee ("clutter/frame-clock: Start next update ASAP after idle period")
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3878>
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