pixels, not (dst+pix)>>1.
This makes the mmx functions bitexact with the C functions.
Originally committed as revision 18527 to svn://svn.ffmpeg.org/ffmpeg/trunk
It contains optimizations that are not specific to i386 and
libavutil uses this naming scheme already.
Originally committed as revision 16270 to svn://svn.ffmpeg.org/ffmpeg/trunk