Improve proc gen

[xen-42]

Feature

Follows on from this branch: https://github.com/Outer-Wilds-New-Horizons/new-horizons/tree/better-proc-gen

Main issue remaining for this to be merged is that the UV generation on the proc gen mesh is done poorly, meaning that we get a huge ugly seam. Fix that and this ought to work.

Context

Proc gen is used on asteroids but can also be used for planets.