Improved mean light formula

Implemented in Indigo in FabricMC/fabric@8a330cc
Fixes some edge cases with smooth lighting, handling discontinuous world light values generally without special casing emissive blocks (which usually cause such discontinuous values) explicitly. See the commit description and pull request description for details on the reasoning behind the changes.

Cubic quad smooth lighting fix

Implemented in Indigo in FabricMC/fabric@d652d86
Fixes fast smooth lighting computation for axis-aligned cubic quads assuming a certain vertex order. See the method javadoc and comments related to GeometryHelper.firstCubicVertex for an explanation of how the fix works.