bvh_pyramid

One level of the pyramid of leaf bounds: each pyramid node folds the bounds of two nodes of the level below (leaves at the first level). Dispatched once per level, the levels let bvh_internal fold the bounds of any run of leaves in a few reads. It reads the leaves of nodes and writes pyr (6 words a node, minimum then maximum) in place, each level reading what the level before it wrote. The params are np, pyr_size and level (1 .. log2(pyr_size)); a dispatch covers pyr_size >> level nodes. The geometry.bvh node runs it once per level, between bvh and bvh_internal. Exact.