https://bugs.musicpd.org/view.php?id=4525

Use libvorbis instead of libvorbisfile, which gives us more control
over the decoding process.

Prepare to reuse the same code for a new Vorbis decoder plugin based
on libvorbis instead of libvorbisfile.

Initialize the OggStreamState as soon as the first page is seen; no
special code needed in the caller.

Required for OpusTags packets which contain artwork.

See https://bugs.musicpd.org/view.php?id=4520

Reduce some overhead.  It is not necessary to copy the object.

av_free_packet() was deprecated in FFmpeg 3.0.