geom: fix interaction of qtbt and octree geometry quantisation
The geometry quantisation assumed that octree nodes were cubes. The addition of qt/bt partitioning modes breaks this premise. This commit addresses the following: - Only the to-be-coded bits, which may differ for each component, are quantised, rather than quantising all bits according to the tree depth (which would result in quantising already coded bits). - The occupancy mask (determining the non-coded occupancy bits) must be recomputed for each node. Since the effect of quantisation is to prune back each sub-tree from the leaves, some nodes that would have had qt/bt partitions will not be coded, leaving an unequal reconstruction. The solution is to terminate the shorter leaves earlier and to recompute the partition.