While reviewing a patch to port Tremor r17541, noticed that the

While reviewing a patch to port Tremor r17541, noticed that the
half-rate decode code in Vorbisfile was never completed past the
original 'quick and dirty' feature request.  Specifically, it made no
attempt to keep the pcm offset tracking consistent in seeks.

Complete this code, and add a testing mode to seeking_example.c to
torture test seeking in halfrate mode.  Also remove requirement that
halfrate mode only work with seekable files.

svn path=/trunk/vorbis/; revision=17560