The dump_extra bitstream filter currently simply adds the extradata to
the packets indicated by the user without checking whether said
extradata already exists in the packets. Besides wasting space
duplicated extradata in the same packet/access unit is also forbidden
for some codecs, e.g. MPEG-2.

This check has been added to be able to use the mpeg2_qsv encoder (which
only adds the sequence headers to the first packet) in broadcast
scenarios where repeating sequence headers are required.

The check used here is not perfect: E.g. dump_extra would add the
extradata to a H.264 access unit consisting of an access unit delimiter,
SPS, PPS and slices.

Fixes #8007.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>