gst/mpegstream/: Fix flow value combination; this fixes playbin/totem locking up...

gst/mpegstream/: Fix flow value combination; this fixes playbin/totem locking up if a VobSub file is specified as sub...

Original commit message from CVS:
* gst/mpegstream/gstdvddemux.c: (gst_dvd_demux_base_init),
(gst_dvd_demux_class_init), (gst_dvd_demux_combine_flows),
(gst_dvd_demux_send_subbuffer):
* gst/mpegstream/gstmpegdemux.c: (gst_mpeg_demux_class_init),
(gst_mpeg_demux_init_stream), (gst_mpeg_demux_parse_packet),
(gst_mpeg_demux_parse_pes), (gst_mpeg_demux_combine_flows),
(gst_mpeg_demux_send_subbuffer):
* gst/mpegstream/gstmpegdemux.h:
* gst/mpegstream/gstmpegparse.c: (gst_mpeg_parse_process_event),
(gst_mpeg_parse_chain):
Fix flow value combination; this fixes playbin/totem locking up if
a VobSub file is specified as subtitle file (#334322). Flow value
combination should only happen once we are fairly sure we've got all
pads that are available for now. Since there isn't a well-specified
time when this is the case in MPEG, we'll just assume this is the
case once there has been a certain number of packets for each
stream we've found so far.