avcodec: add C xvid IDCT support

Thanks to Pascal Massimino and Michael Militzer for permission to use under LGPL

The xvid idct code is from xvid, and nearly unchanged to make future syncing easy
the integration into ffmpeg is done by the commiter
the commit message is written by the commiter
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>