Dynamic BVH for empty areas

[Aksel Hjerpbakk]

Use software Bounding Volume Hierarchy (and later hardware accelerated BVH for hardware that support it) for empty areas of the grid.

A empty voxel should be able to point to a BVH entry that defines empty space to accelerate traversal

Relevant article:
https://jacco.ompf2.com/2022/06/03/how-to-build-a-bvh-part-9a-to-the-gpu/