Dispatch jitter is how much the dispatch interval has changed between
frames. It's a measure of sampling smoothness for events that are occurring
at a higher rate than the screen is refreshing:
* Mouse movement
* Clients rendering at swap interval zero
* Keyframe animation position
Zero jitter is ideal but will practically never happen, and a jitter value
of several thousand microseconds will be visible to the naked eye as stutter
even if you're maintaining a perfect frame rate.
To make the numbers easier to interpret we also log the jitter as a
percentage of the refresh interval.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3082>
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