OPtimized case of seeking within a single link; reset decode state
without dumping decode setup. Roughly 5-10x faster this way.
Raft of small vorbisfile seek fixes (nearly all to do with exact
positioning around stream boundaries).
git-svn-id: http://svn.xiph.org/trunk/vorbis@4404 0101bb08-14d6-0310-b084-bc0e0c8e3800