[Cook-Torrance] Investigate the symmetric Fresnel calculation

[Razakhel]

It has been pointed out that Cook-Torrance's most common explanations don't include the fact that the Fresnel component is actually symmetric.

See:

• Earl Hammon's GDC talk, particularly from slide 99 to 108, although many slides prior to these are worth looking at.
• This good short explanation post: https://computergraphics.stackexchange.com/a/2289/9321.

[devshgraphicsprogramming]

don't forget the 1.05 correction factor for diffuse internal scatter only applies to glass with a schlick approximation, the correction factor we've derived is not yet published anywhere (if you require we can make the Sage Math workbook available)

[Razakhel]

No worries, I don't have time right now to actually investigate it, and most of the investigation will be on Hammon's slides anyway, so no rush 🙂 thanks for pointing that out though!