BRDF improvements

[vorg]

Clarify use of D_GGX and F_Schlick

[dmnsgn]

Our current BRDF model:

Layer	Distribution (D)	Visibility (V)	Fresnel (F)
Specular isotropic	D_GGX	V_Smith_GGX_Correlated	F_Schlick (f90 = 1)
Specular anisotropic (TODO)	D_GGX_ANISOTROPIC	V_GGX_ANISOTROPIC	F_Schlick (f90 = 1)
Clearcoat	D_GGX	V_Neubelt	F_Schlick (f0 = 0.04, f90 = 1)
Sheen	D_Charlie	V_Charlie	F = 1

But we have duplicates and here are my improvement suggestions:

Specular isotropic layer:

• F:
  Currently using HdotV (is that implementation from MaterialX?).
  It is the same as F_Schlick.
  => Use improved F_Schlick from filament instead

• D:
  Currently using MicrofacetDistribution function GGX, Trowbridge-Reitz.
  It is the same as D_GGX (used by clearcoat).
  => Use D_GGX

Specular anisotropic layer: implement

Iridescence: find out model