MMX & MMX2 optimizations (MMX2 is buggy and commented out)

32, 24, 16, 15 bit support in C (only 32 & 16 tested)
32, 16 bit support in mmx (16 with dithering)
ranges of some variables changed so mmx likes them more


git-svn-id: file:///var/local/repositories/mplayer/trunk/postproc@2233 b3059339-0415-0410-9bf9-f77b7e298cf2