ext/ffmpeg/gstffmpegdec.c: Fix timestamping for out-of-order incoming buffers. Instead of blindly copying the incomin...

Original commit message from CVS:
* ext/ffmpeg/gstffmpegdec.c: (gst_ffmpegdec_init),
(gst_ffmpegdec_open), (gst_ffmpegdec_save_incoming_values),
(gst_ffmpegdec_get_best_values), (gst_ffmpegdec_video_frame),
(gst_ffmpegdec_sink_event):
Fix timestamping for out-of-order incoming buffers. Instead of blindly
copying the incoming buffer timestamps on the outgoing buffers we cache
the latest 2 incoming buffer timestamps and duration and make a wise
choice as to what the outgoing buffer timestamp and duration should be.
Fixes #342962