ext/ffmpeg/gstffmpegdec.c: Detect DTS or PTS as timestamps. This is done by tracking frame reordering on the output a...

Original commit message from CVS:
* ext/ffmpeg/gstffmpegdec.c: (gst_ffmpegdec_class_init),
(gst_ffmpegdec_setcaps), (check_keyframe),
(gst_ffmpegdec_video_frame), (gst_ffmpegdec_sink_event),
(gst_ffmpegdec_set_property):
Detect DTS or PTS as timestamps. This is done by tracking frame
reordering on the output and making sure that timestamps don't go
backwards. Fixes #482660.