PBVH_FACES and PBVH_GRIDS do not store faces directly in nodes;
instead they store 'primitives', which are tesselation triangles
for PBVH_FACES and grids (which are per-loop) for PBVH_GRIDS.
Primitives from the same face could sometimes end up in different
PBVH nodes. This is now prevented in two ways:
* All primitives of the same face are given the same boundary
during PBVH build. This prevents them from being swapped
away from each other during partitioning.
* build_sub adjusts the final partition midpoint to fall
between primitives of different faces.
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