livesync: Use the actual output buffer duration of gap filler buffers
Otherwise the following can happen:
- 25fps stream
- buffer with PTS 0ms, duration 20ms arrives, is output
- buffer with PTS 40ms, duration 20ms arrives
- is considered early because 20ms < 40ms
- filler buffer with PTS 20ms and 40ms duration is output
- buffer with PTS 40ms is output
After this change no filler would be inserted because the gap is smaller than the duration of a filler buffer.
Also, previously the 40ms duration would be used if a filler was previously output because in that case the cached output buffer duration would've already been patched from 20ms to 40ms.