avcodec: Rename xvidmmx IDCT to xvid

The Xvid IDCT is not MMX-specific.
Merge commit 'd35b94fbabd8beb5d566c0b5d01688aff62c3b36'

* commit 'd35b94fbabd8beb5d566c0b5d01688aff62c3b36':
  avcodec: Rename xvidmmx IDCT to xvid

Conflicts:
	doc/APIchanges
	libavcodec/version.h

Merged-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/avdct: add "xvid" alias AVOption and use FF_IDCT_XVID

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