There are lots of files that don't need it: The number of object
files that actually need it went down from 2011 to 884 here.
Keep it for external users in order to not cause breakages.
Also improve the other headers a bit while just at it.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
The contents of this field are not defined for decoding. Use
pkt_timebase, which is the timebase of demuxed packets.
Drop a tautological av_packet_rescale_ts() call, as the stream and
decoder timebases are the same.
