ext/ffmpeg/: Use special functions for timestamp-conversion again, since we otherwise don't handle clocktimenone corr...

Original commit message from CVS:
* ext/ffmpeg/gstffmpegcodecmap.h:
* ext/ffmpeg/gstffmpegdec.c: (gst_ffmpegdec_connect),
(gst_ffmpegdec_frame), (gst_ffmpegdec_chain):
* ext/ffmpeg/gstffmpegdemux.c: (gst_ffmpegdemux_loop):
* ext/ffmpeg/gstffmpegenc.c: (gst_ffmpegenc_link),
(gst_ffmpegenc_chain_video):
* ext/ffmpeg/gstffmpegmux.c: (gst_ffmpegmux_loop):
Use special functions for timestamp-conversion again, since we
otherwise don't handle clocktimenone correctly (#311593).