Optimize split MC, so we don't always do 4x4 blocks of 4x4pixels each, but

Optimize split MC, so we don't always do 4x4 blocks of 4x4pixels each, but
we apply them as 16x8/8x16/8x8 subblocks where possible. Since this allows
us to use width=8/16 instead of width=4 MC functions, we can now take more
advantage of SSE2/SSSE3 optimizations, leading to a total speedup for splitMV
filter of about 10%.

Originally committed as revision 23853 to svn://svn.ffmpeg.org/ffmpeg/trunk