gst/realmedia/rmdemux.c: Fix video timestamps by adjusting it with the first timestam...

gst/realmedia/rmdemux.c: Fix video timestamps by adjusting it with the first timestamp found.

Original commit message from CVS:
* gst/realmedia/rmdemux.c: (gst_rmdemux_parse_mdpr),
(gst_rmdemux_fix_timestamp), (gst_rmdemux_parse_video_packet),
(gst_rmdemux_parse_audio_packet), (gst_rmdemux_parse_packet):
Fix video timestamps by adjusting it with the first timestamp found.
Don't assume we have a complete fragment when flushing the adapter,
packets might have been lost or the stream might just be broken.