Merge commit 'a786c825'

* commit 'a786c825':
  idct: Split off Xvid IDCT

Conflicts:
	libavcodec/Makefile
	libavcodec/mpeg4videodec.c
	libavcodec/x86/Makefile
	libavcodec/x86/idctdsp_init.c

This split is somewhat restructured leaving the xvid IDCT available
outside mpeg4 if manually selected.

The code also could not be merged unchanged as it conflicted with a
bugfix in FFmpeg

Merged-by: Michael Niedermayer <michaelni@gmx.at>