ext/ogg/gstoggdemux.c: Use GstSegment infrastructure to remove duplicated code and handle more seek cases correctly.

Original commit message from CVS:
* ext/ogg/gstoggdemux.c: (gst_ogg_demux_receive_event),
(gst_ogg_pad_event), (gst_ogg_pad_internal_chain),
(gst_ogg_demux_chain_peer), (gst_ogg_pad_submit_packet),
(gst_ogg_demux_deactivate_current_chain),
(gst_ogg_demux_activate_chain), (gst_ogg_demux_do_seek),
(gst_ogg_demux_perform_seek), (gst_ogg_demux_collect_info),
(gst_ogg_demux_find_chains), (gst_ogg_demux_chain),
(gst_ogg_demux_loop), (gst_ogg_demux_change_state):
Use GstSegment infrastructure to remove duplicated code
and handle more seek cases correctly.