demux_ogg: Hacky, not-quite-correct support for Opus.

Not correct in the meaning that the FFmpeg decoder at least
complains about some invalid packets.
Have not noticed any real issues in short testing though.

git-svn-id: svn://svn.mplayerhq.hu/mplayer/trunk@37304 b3059339-0415-0410-9bf9-f77b7e298cf2