oggz uses reverse calculation for vorbis granulepos when forward calculation would work

When using liboggz with the OGGZ_AUTO option to reconstruct missing granulepos entries, reconstruction for Vorbis audio packets is always done using the reverse calculation from the last packet in a page, and never in the forwards direction.

In slow processing environments (JavaScript cross-compilation on an old 32-bit iPad or iPhone) this can cause demuxing to sometimes spend a couple extra milliseconds processing through the queued future packets doing reverse calculation, from the explicit granulepos at the end of a page back to the first packet in the page.

The forward calculation was disabled in this 2007 commit:

http://git.xiph.org/?p=liboggz.git;a=commit;h=7f016220d96f98dae0519d94bcf88323c002e656

by adding an early 'return -1' into auto_calc_vorbis.

I'm not sure if it was supposed to do that (why keep the unreachable code?) or if that was a test for debugging that got committed by mistake.

If I manually remove the 'return -1', the forward reconstruction happens one packet at a time as I expect it, but I don't know if there's anything else wrong with that formerly-unreachable code.