Convert Tremor to use libogg instead of its own internal libogg2.

This makes it easier to use Tremor as a drop-in replacement for libvorbis and
 reduces code size and overhead for those who don't want to use its built-in
 Ogg demuxer.
This commit also backports all of the changes that have accumulated in
 libvorbis's vorbisfile implementation, with the exception of halfrate decoding
 and cross-lapped seeking.
Those should not be too hard to add if someone really wants them.


git-svn-id: https://svn.xiph.org/trunk/Tremor@17375 0101bb08-14d6-0310-b084-bc0e0c8e3800