libxvid: Create extradata in init using a dummy frame

Modifying global header extradata in encode_frame is an API violation
and only happens to work currently because mov writes its header
at the end of the file.

Heavily based off of a patch from 2012 by Nicolas George.

Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>
