Optimize memset_c and copy_c a bit

Use only one call to memset/memcpy in common case when pitch == width
Rewrite mt_motion asm
Move ugly _mm_set1_epi32 to a function
Remove motionmask.asm

Also fix misleading variable name in mask_sse2_op (wMod32 -> wMod16)
Rewrite mt_merge(luma=false) asm
Rewrite mt_merge(luma=true) asm