The clutter_get_current_event_time() function will return the event
timestamp coming from a Clutter event. Clutter might synthesize or
throttle events, so the function cannot be used when dealing with
backend-specific use cases.
The X11 backend is the only backend supported by Clutter that makes
use of timestamps, so it's altogether fitting that it should come
with a specific function to deal with the timestamps of the X events.
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