Alternative BRDF's/shading models for voxel layers

[heyx3]

A different issue mentions transparency, which is a bit more complex than a mere change of lighting model.

This change would require a smarter deferred lighting pass, using different models per surface. Consider using the stencil buffer to store a surface's lighting model.

• Multi-layered surfaces: https://belcour.github.io/blog/slides/2018-brdf-realtime-layered/slides.html#/