VisibilityExtimator ("Occlusion Culling" / Cave Culling)

[BrentDaMage]

Currently caves are not being culled properly as they are in Java, due to a lack of occlusion culling support in all GFX APIs. If we were to pull this class from 0.12.1, this will allow us to have caves present in the world without taking a significant performance hit.

[BrentDaMage]

Should actually be renamed to VisibilityEstimator.

[BrentDaMage]

Here's a blogpost describing what we should be doing: https://tomcc.github.io/2014/08/31/visibility-1.html