usage of ABSDIFF macro in flow.c

[mike239x]

The commit 24b790d states:

Replace SAD function with assembly version ~ 25% faster

However ABSDIFF macro (the assembly version) uses hardcoded image width of 64 px, while compute_sad_8x8 has variable image width. So I made an experiment - I added timing + debug output to the code and measured compute_sad_8x8(..., (uint16_t) FRAME_SIZE) vs ABSDIFF(...) vs compute_sad_8x8(..., 64); and I got that the last one is actually the fastest. Also, the speed up from compute_sad_8x8(..., (uint16_t) FRAME_SIZE) to ABSDIFF(...) is far less than 25% for me - more like 8%.
Assuming somebody can reproduce my results - maybe we can just delete ABSDIFF macro and use compute_sad_8x8(..., 64) instead?