cf5b659bc6
Transition from PLAYING to PAUSED unschedules any pending clock wait, but there was no guard that prevented another input buffer then waiting again while in PAUSED before dataflow stops. Use a new `playing` flag to avoid this, and exit the chain function if instead of playing we're now flushing. Basically the same as the clocksync element. Closes https://gitlab.freedesktop.org/gstreamer/gstreamer/-/issues/1772 Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/4098>