By removing the Fresnel term (often ommitted from fabric/cloth BRDFs)
we can store the DG term for the cloth BRDF in the 3rd channel of the
existing DFG LUT.
* 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
