ext/: More seeking fixes, oggdemux now supports seeking to time and uses the downstream element to convert granulepos...

Original commit message from CVS:
* ext/ogg/gstoggdemux.c: (gst_ogg_demux_init),
(gst_ogg_demux_get_formats), (gst_ogg_demux_src_query),
(gst_ogg_demux_src_event), (gst_ogg_demux_src_convert),
(gst_ogg_demux_handle_event), (gst_ogg_demux_seek_before),
(_find_chain_get_unknown_part), (_find_streams_check),
(gst_ogg_demux_push), (gst_ogg_pad_push):
* ext/theora/theoradec.c: (theora_get_formats),
(theora_dec_src_convert), (theora_dec_sink_convert),
(theora_dec_src_query), (theora_dec_src_event), (theora_dec_event),
(theora_dec_chain):
* ext/vorbis/vorbisdec.c: (vorbis_dec_get_formats),
(vorbis_dec_convert), (vorbis_dec_src_query),
(vorbis_dec_src_event), (vorbis_dec_event):
More seeking fixes, oggdemux now supports seeking to time and
uses the downstream element to convert granulepos to time.
Seeking in theora-only ogg files now works.