BRDF Correctness

[MilchRatchet]

The current material workflow, especially the weighing of directions, is shotty and probably mostly inaccurate. (Fog is so bad I don't even want to start talking about it)

There are material models that combine specular and diffuse BRDF. This probably better than the current approach. The current BRDF has quite a significant energy loss.

[MilchRatchet]

The BRDF was massively improved in 416f981. However, there is now also a new bug with metallic materials with almost 1 albedo. The refraction BRDF looks mostly fine but it is probably worth taking a closer look once more.

These two things are the remaining things for this issue. The fog will be handled in another issue.

[MilchRatchet]

The bug with metallic materials is somewhat fixed in a917341. The issue is that the multiscattering compensation assumes valid reflection which is not always the case with light sampling. A more rigid solution may be good here but that is something for the future.

[MilchRatchet]

I reworked the refraction BRDF a bit in 388f5fa. It is now conserving more energy and takes albedo into account. It is still not a good implementation but since refraction is only used by the toy and the ocean, it is less important and thus I will close this issue for now.