clutter/frame-clock: Ignore missing presentation timestamps
From the commit message:
In some hardware configurations, presentation timestamps could be
missing from some page flip events, leading to a temporary loss of
vblank synchronization.
This occurs at least with AMD GPUs for atomic commits that only update
the cursor plane. [0]
In those cases, it's better to calculate the next update time
according to the last valid presentation timestamp instead of relying
on the dispatch lateness.
[0] https://gitlab.freedesktop.org/drm/amd/-/issues/2030