Fix calculation of unit_idx to identify Av1RestorationUnit

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.

Fixes the valgrind warnings I saw in the sample from #173 (closed). I was not able to reproduce the actual bug.