the count of decodeable frames, instead of the index. This is similar
to what other codecs are doing, and in theory conceptually simpler.

This change was originally proposed by Thomas Vander Stichele.

Since this is a bitstream change, the stream version number has
been bumped to 3.2.1. The decoder will correctly support both
the older 3.2.0 granulepos scheme and the new one. By spec,
old decoders should attempt to play 3.2.1 streams, but sync
will be off by one in the less noticeable direction.

Patch from Maik Merten.

svn path=/trunk/theora/; revision=13856