In practice it worked fine, but only by accident. NaN's were
being passed to the lerp_slice function, which led to the
correct result in this case but is icky and dependant
on how lerp_slice is implemented.
The BVH building code is now largely split out into a separate
type, BVHBase. The intent is that this will also be used by
the BVH4 when I get around to it.
The BVH itself now uses references instead of indexes, allocating
and pointing directly into the MemArena. This allows the nodes
to all be right next to their bounding boxes in memory.
