Fresnel model

At the Physically Based Shading course at Siggraph this year Naty Hoffman presented issues with established Fresnel models. Both how it's incorrect to use the physical Fresnel equations directly in an RGB renderer, and also how Gulbrandsens method for making the parameterization artist friendly in fact fails at this on several counts.
[slides](https://blog.selfshadow.com/publications/s2020-shading-course/hoffman/s2020_pbs_hoffman_slides.pdf)

In addition Laurent Belcour presented a new Fresnel model, that is more accurate than Schlick's approximation and still works well for both offline and real-time rendering.
[slides & code](https://belcour.github.io/blog/research/publication/2020/08/26/brdf-fresnel-decompo.html)

Since Standard Surface is using Gulbrandsens methods, and definitely needs to be usable and accurate for non-spectral renderers, this is something we need to look into. Should we move away from Gulbrandsens method? And if so, what should we use instead?

Feedback from the rendering community is much appreciated!