ext/ffmpeg/gstffmpegdec.c: Backported some fixes from 0.8. Most importantly, use demuxer's timestamping information i...

Original commit message from CVS:
* ext/ffmpeg/gstffmpegdec.c: (gst_ffmpegdec_init),
(gst_ffmpegdec_close), (gst_ffmpegdec_open),
(gst_ffmpegdec_setcaps), (gst_ffmpegdec_negotiate),
(gst_ffmpegdec_frame), (gst_ffmpegdec_sink_event),
(gst_ffmpegdec_chain):
Backported some fixes from 0.8. Most importantly, use
demuxer's timestamping information if we use its framerate
value. Makes divx play properly again among other things
(#319616, #313970, #317596). Did not backport memory leak
fix.