Original commit message from CVS:
* ext/ffmpeg/gstffmpegdec.c: (gst_ffmpegdec_video_frame),
(gst_ffmpegdec_audio_frame), (gst_ffmpegdec_frame),
(gst_ffmpegdec_sink_event), (gst_ffmpegdec_chain):
More timestamp fixage.
Don't pass around buffers just to pass its timestamp since we
cannot update it.
After decoding a buffer, don't use the timestamp of the buffer
anymore but let the interpolation do its work.