1. Processing each l-th primitive at the binning step, where
l=log10(N) and N is number of primitives in the current node.
2. Experiments demonstrated that it is beneficial to switch to
exact SAH computation when the number of primitives in the current
node is less or equal to the number of bins.
3. Experiments show that using the same fixed number of 32 bins at
any level is sufficient.
4. Re-binning with 32 * 2^N, N is the tree depth.