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 refraction, default to 1.5.
	When we add a coat layer, we need to recalculate the reflectance of the interface between
	the base material and the exterior, as the new exterior is now the coat
	layer. The precomputation of the reflecance is done in
	vtkOpenGLPolyDataMapperNode.

It is possible to define a normal map for the coat layer. If not
presents, we assume the layer has the same normal map as the base
material.