Vorbisfile API addition for game and mod coders;

Do a 'free' sample rate conversion from source rate to half source
rate by calling ov_halfrate() after ov_open(); Although this is solid
for immediately desired usage, I want to clean it up a bit before
advertising it's existence, so no doc addition yet.

Does not break binary API.

Monty

svn path=/trunk/vorbis/; revision=5222