Feature/per region octree

[NotStirred]

merge after #1256 and #1257

Mainly looking for comments on overly verbose or confusing stuff, or better approaches to what I'm doing. There is a lot missing from this PR currently (multithreading, per-region finalisation, scene abstraction) You only need to look at 2e7b09d

This makes CubicChunks support quite a pita, I plan do some scene refactoring to allow for some abstraction in the chunk loading steps

This PR is relatively complex, and some of the logic is a little hard to follow. I intend to comment it more to explain some of it.

It introduces the concept of a sub-octree which is a (at maximum) 512^3 size octree. All of the sub-octrees for the region are loaded, and then they replace an entire node in the global octree

I hope to add finalisation to this also, and then finalise the seams once all regions are loaded.

[NotStirred]

Still to do for this pr:

• Per-region octree finalisation, where the inside of each octree is finalised and the seams are finalised after merging
• Option to disable the per-region octree mode to reduce the size of the octree