This takes into account whether the granule defines the start or end times
of packets, and sets the correct file offset of the associated page.
Originally committed as revision 22462 to svn://svn.ffmpeg.org/ffmpeg/trunk
Otherwise it gets set automatically to a page midstream and prevents seeking
to the first page.
Originally committed as revision 22454 to svn://svn.ffmpeg.org/ffmpeg/trunk
David Conrad
Måns Rullgård