It contains the x264 version info and options used for encoding
If we let it in extradata it will not be present in the final file

libavcodec does that already, and this makes it easier to compare x264
options used by VLC and FFmpeg.

Comparing FFmpeg and VLC with the exact same set of options might help
finding why mpeg (at least mpeg2/mpeg4/flv) encoding is 5 times slower
with vlc than with ffmpeg, supposedly for the same set of options.