mux/ogg: Enable OggDirac support

 - OggDirac support using revised granule_position mapping:
     http://www.diracvideo.org/specification

 (20081109)
 - Uses p_extra for BOS page
 - Flushes Ogg page after each OggDirac packet

Signed-off-by: David Flynn <davidf@rd.bbc.co.uk>
Signed-off-by: Laurent Aimar <fenrir@videolan.org>