avformat/movenc: creating producer reference time (PRFT) box

avformat/movenc: creating producer reference time (PRFT) box

The producer reference time box supplies relative wall-clock times
at which movie fragments, or files containing movie fragments
(such as segments) were produced.
The box is mainly useful in live streaming use cases. A media player
can parse the box and utilize the time fields to measure and improve
the latency during real time playout.