The bug was in the previous commit, where I thought I was
preventing out-of-bounds access during traversal by limiting
the tree depth. While the idea was correct, I forgot that the
traversal stack needs _2_ extra slots on top of the tree depth,
not just 1. Fixed.
This avoids exceeding max BVH depth even in pathological cases.
Still need improve non-worst case building, but this at least
prevents crashes in worst case.
