Suboptimal utilization of cached shadow buffers.

[bryanedds]

An algorithm required to more fully utilize the cached shadows is currently beyond my understanding. I attempted one approach that involved swapping said buffers (and their cache-related bookkeeping) in a way that mostly maximized utiliztaion in this branch -

https://github.com/bryanedds/Nu/tree/shadow-caching

Unfortunately, due to how sensitive the approach is to the sort of mutational fix-ups I applied, I never could get it to work reliably (EG, it ended up swapping things in a seemingly incorrect manner).

If we could find some algorithm that could facilitate better cached shadow utilization, that would get us up to expectations.