If an update (new frame) had been scheduled already before
`_clutter_stage_cogl_presented` was called then that means it was
scheduled for the wrong time. Because the `last_presentation_time` has
changed since then. And using an `update_time` based on an outdated
presentation time results in scheduling frames too early, filling the
buffer queue (triple buffering or worse) and high visual latency.
So if we do receive a presentation event when an update is already
scheduled, remember to reschedule the update based on the newer
`last_presentation_time`. This way we avoid overfilling the buffer queue
and limit ourselves to double buffering for less visible lag.