Better approximation for darkening layers under coat

Due to the possibility of total internal reflection at the coat/external medium boundary, layers under the coat should be darker. See https://graphics.cs.yale.edu/sites/default/files/wet.pdf for a detailed model describing the various effect of having a water layer on top of materials. I think that interpenetration of layers is out of scope for OpenPBR surface, but the total internal reflection should be modelled.

In the OpenPBR specification we describe the phenomenon without providing implementation details :
`In the full light transport this observed color is further darkened and saturated due to multiple internal reflections from the inside of the coat, including a considerable amount of total internal reflection, which causes light to strike the underlying material multiple times and undergo more absorption. Also the observed tint color should vary away from coat_color as the incidence angle changes, due to the change in path length in the medium. The presence of a rough coat will increase the apparent roughness of the BSDF lobes of the underlying base. We generally assume that in the ground truth appearance, all these effects are accounted for.`

In Standard Surface there is a dedicated parameter to simulate the darkening of layers under coat using the following formula :
`base_color = pow(base_color, 1.0 + (coat * coat_affect_color)) subsurface_color = pow(subsurface_color, 1.0 + (coat * coat_affect_color))
`
This approximation is simple and disabled by default in Standard Surface.

In OpenPBR we should evaluate it against ground truth, possibly find a better approximation, and decide whether we want to make the effect user controllable.