libavutil: document side effects of macros

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
hevc: use intreadwrite

When dealing with MVs, both components may be processed at a time.

On Win64, 560 to 539 cycles for derive_spatial_merge_candidates.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
Merge remote-tracking branch 'ffmpeg-upstream/master' into ffmpeg_patch