avformat/mpeg: Make VobSub demuxer have its own context struct

avformat/mpeg: Make VobSub demuxer have its own context struct

When the VobSub demuxer was added, the fields it required were simply
added to the MpegDemuxContext (if the VobSub demuxer was selected at
all). The mpeg demuxer of course doesn't use these fields even if they
are there; and the VobSub demuxer doesn't use the old ones: It opens an
mpeg subdemuxer of its own and uses this where a mpeg demuxer is
required. Hence the two contexts can be split, saving memory.

Furthermore several headers can now be moved to the section that is
guarded by #if CONFIG_VOBSUB_DEMUXER (this even includes avassert.h
which was unguarded and has been added in 9cde9f70 despite not being
used in that patch).

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