Read correct number of loop restoration coeffs in each superblock

As specified in section 5.11.57 in the AV1 spec, the LR coefficients read for a superblock are the ones for LR units whose top-left corner lands in the superblock.

The code was assuming that there was exactly one LR unit per superblock, which is not always true (in particular, some tiles may have no LR units).

This fixes issue #180 (closed)