Ronald S. Bultje authored Nov 16, 2018 and Janne Grunau committed Nov 16, 2018

ruy and rux are in unit_size dimensions, whereas lr_mask are in sb128
dimensions, and unit_idx is in sb64 dimensions, so one can't be derived
from the other. Instead, remove ruy/rux and derive unit_idx and sb_idx
directly from the block positions aligned to the unit_size.