The 4-tap filters should only access one row/column before the
reference block.
Signed-off-by: Mans Rullgard <mans@mansr.com>
(cherry picked from commit e0e46cae37)
mm_support() instead.
Reduce complexity and simplify pending move to libavutil.
Originally committed as revision 25074 to svn://svn.ffmpeg.org/ffmpeg/trunk