[libvorbis] Bugfix: seek broken for large files and on pure 32 bit architectures

In file vorbisfile.c, function

   int ov_pcm_seek_page(OggVorbis_File *vf,ogg_int64_t pos)

the calculation of the approximate seek point causes an overflow for
very large files and on pure 32 bit architectures.

   /* take a (pretty decent) guess. */
   bisect=begin +
     (target-begintime)*(end-begin)/(endtime-begintime) - CHUNKSIZE;

The temporary expression (target-begintime)*(end-begin) grows with the
square of the file size. This effectively limits the file size to the
square root of the maximum value of ogg_int64_t. In consequence the file
is scanned from the very beginning which is pretty slow over a network.


FIX:

   bisect=begin +
(ogg_int64_t)((double)(target-begintime)*(end-begin)/(endtime-begintime))
- CHUNKSIZE;

Since it is an approximation anyway, the calculation should be done
using floating point arithmetics.


Marcel