MMX2 code, because the HAVE_ defines do not have sane values here if
runtime cpudetection is used. Hopefully fixes runtime cpudetection
breakage from r19339.


git-svn-id: file:///var/local/repositories/mplayer/trunk/libswscale@19374 b3059339-0415-0410-9bf9-f77b7e298cf2