Pass the composition and ancillary ID for DVB subtitles via extradata instead
of sub_id, this allows detecting when that information is not available and
just decode everything.
In addition extradata is required for many codecs and thus in contrast to
sub_id generally already passed on by any programs using libav*.
Also ask for a sample if we encounter a stream with multiple/changing IDs.
Originally committed as revision 24238 to svn://svn.ffmpeg.org/ffmpeg/trunk