Add a granule to granulepos conversion function.  Fix the duration
function for vorbis.  Handle timestamps on header packets differently
and be more careful about calculating OFFSET and OFFSET_END.  After
this change, timestamps for vorbis don't exactly match up with the
timestamps that vorbisparse outputs, but it's unclear if vorbisparse
is actually correct and it would add a lot more code to make oggdemux
match vorbisparse.  Fixes #602790.