perform interpolation steps in such a order so that halfpel interpolation
could be done per picture
this also make mc_block() match h.264 for the 1/4 pel cases so that the
use of the h264 functions for some cases doesnt inroduce a fantastic mess


git-svn-id: file:///var/local/repositories/ffmpeg/trunk@10433 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b