Merge branch 'cycles_disney_brdf' into cycles_disney_bsdf_transmittance

Removed reflection call when roughness is low because of artifacts.

Enhanced the thin surface specular transmission approximation by a simple reflection to the other side without refracting it (refraction approximately cancels for thin surfaces).

Indication if to use fresnel is now handled via the type of the BSDF.

Better roughness remapping for thin surface specular refractions.

Implemented the specular transmission part of the thin surface
approximation.

Implemented the thin surface diffuse part.

Fixed an error in the clearcoat where it appeared too bright for default
light sources (like directional lights)

Resolved inconsistencies in using tabs and spaces

Improved the clearcoat part by using GTR1 instead of GTR2

Use reflection BSDF for glossy reflections when roughness is 0.0 to
reduce computational expense and some code cleanup

Code cleanup includes:
- Code style cleanup and removed unused code
- Consolidated code in the bsdf_microfacet_multi_impl.h to reduce
some computational expense

Fixed glossy reflections and refractions for low roughness values and
cleaned up the code.

For low roughness values, the reflections had some strange behavior.

Removed default values in setup functions and added extra functions for
GGX with fresnel.

Switched from uniform to cosine hemisphere sampling for the diffuse and
the sheen part.

Removed the color parameters from the diffuse and sheen shader and use
them as closure weights instead.

Fixed the issue with artifacts when using anisotropy without linking the
tangent input to a tangent node.

Added subsurface radius parameter to control the per color channel
effection radius of the subsurface scattering.

Rearranged the inputs of the shader.

Put spaces in the parameter names of the shader node

Removed code that isn't in use anymore