ext/ffmpeg/: Makes ffmpegdec and ffmpegenc use these functions and remove a comment from a previuos patch

Original commit message from CVS:
* ext/ffmpeg/gstffmpegenc.c (gst_ffmpegenc_chain_video):
* ext/ffmpeg/gstffmpegdec.c (gst_ffmpegdec_chain):
Makes ffmpegdec and ffmpegenc use these functions
and remove a comment from a previuos patch

* ext/ffmpeg/gstffmpegcodecmap.h:
Add two utility functions to convert the pts from/to
the ffmpeg format