Add node for directionally varying emission.

[niklasharrysson]

This change list adds a new node to pbrlib: generalized_schlick_edf.

  <!--
    Node: <generalized_schlick_edf>
    Modifies an EDF with a directional factor. Attenuates the emission distribution of the base EDF according to
    a generalized Schlick fresnel function.
  -->
  <nodedef name="ND_generalized_schlick_edf" node="generalized_schlick_edf" nodegroup="pbr" doc="Modifies an EDF with a directional factor.">
    <input name="color0" type="color3" value="1.0, 1.0, 1.0" />
    <input name="color90" type="color3" value="1.0, 1.0, 1.0" />
    <input name="exponent" type="float" value="5.0" />
    <input name="base" type="EDF" value="" />
    <output name="out" type="EDF" />
  </nodedef>

This node can be used to create a directionally varying / Fresnel-like modulation to emission, using the generalized Schlick Fresnel curve. The main idea is to use this to approximate how emission is attenuated when placed under a coating, which is supported in shading models like Standard Surface.

Using the new node the graph implementation of Standard Surface is updated, to address this as reported in #1057.

Note that this is a simple solution that only account for the Fresnel effect. The roughness of the coat, or multiple scattering in the coat layer, is not considered. We are limited to what can be represented in all the shading languages that MaterialX needs to support. Directionally varying emission was not supported in MDL util recently, and this new node matches what can be expressed in MDL (from version 1.7).

Simple example to show the node result, here modulating a uniform white emission with yellow->red for facing->gracing angles:

Example to show the improved implementation of Standard Surface (top = old implementation, bottom = new implementation):

[niklasharrysson]

Comparing GLSL (top) with OSL (bottom):

[jstone-lucasfilm]

This looks great to me, thanks @niklasharrysson!