code cleanup. also, set up mag argmax mask. (but, not sure how i want to handle the actual atan2)
doiii, there's an _mm_abs_epi16
cleanup old max_epi16 stuff...found that there's a real instruction for it
select gradX[max mag channel], gradY[max mag channel].
cleanup old stuff
cleanup old 'cvt 8 to 16' ... now using unpackhi, unpacklo