ext/: Try to fix seeking problems in OGM files, especially those containing non-continuous data streams (a.o. #171777).

Original commit message from CVS:
* ext/ogg/gstoggdemux.c: (gst_ogg_demux_push), (gst_ogg_sync),
(gst_ogg_pad_push):
* ext/ogg/gstogmparse.c: (gst_ogm_parse_init),
(gst_ogm_parse_kick_event), (gst_ogm_parse_chain):
* ext/pango/gsttextoverlay.c: (gst_textoverlay_event),
(gst_textoverlay_loop):
Try to fix seeking problems in OGM files, especially those
containing non-continuous data streams (a.o. #171777).