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