inputselector: Avoid deadlock when shutting down

inputselector: Avoid deadlock when shutting down

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/4099>