* Add clear coat normal map support
This change allows to set a separate normal map for the clear coat layer of a material.
* Document the new clearCoatNormal property
* Fix typo
* Document better BRDFs
- Document the loss of energy in the specular BRDF
- Start documenting the gain of energy in the diffuse BRDF
* Explain energy compensation with second scaled lobe
* Add multiscattering pre-integration
* Add new Cloth NDF
Based on the work by Estevez and Kulla, also known as the "Charlie" sheen.
* Restore DFG computation to GGX
* New proposed approximation for Cloth DFG
* Add "Charlie" cloth DFG approximation to cmgen
* Switch to "Charlie" sheen for cloth shading
* Document the new cloth NDF and DFG approximation
