Area of lights and spherical light distribution

[ykafia]

Part of the implementation of Stride's light distribution is based on frostbite's PBR implementation which also talks about disk shaped lights and other kind of implementation. I read in some of the shader comments that the spot light was supposed to have more shapes (as the folks at frostbite were explaining in this presentation) but might have stalled.

If we use that + linearly transformed cosine for BRDF we can implement a more efficient version of light areas.