Update PBR shader in vtkOpenGLPolyDataMapperNode to simulate a clear
coat layer (upside layer on the material). This layer is supposed
dielectric and with low roughness, so it adds only a specular lobe to
the previous diffuse and specular parts of the base material.
The specular BRDF model of the coat layer is a Cook-Torrance specular
microfacet model,
with a GGX normal distribution function, a Smith visibility function,
and a Schlick
Fresnel function. Notice that it uses the exact same specular microfacet
model as the base layer specular BRDF.

It adds multiples parameters to vtkProperty :

* CoatStrength : [0-1], default to 0.0. Control the strength (0 means no layer)
* CoatRoughness : [0-1], default to 0.0. 0 means smooth layer (high specular
  reflection)
* CoatIOR : [0-4], default to 2.0. Define the material IOR of the layer.
* CoatColor : Default to white.
* CoatNormalScale : Scale the normal map (if present). Default is 1.0
* BaseIOR : the base material index of...