Request: Support for KHR_materials_retroreflection

The Khronos PBR TSG is preparing a new draft called KHR_materials_retroreflection:

https://github.com/KhronosGroup/glTF/pull/2610

We think this should be a simple change to the Sample Renderer's PBR shader. The metallic & dielectric specular lobe of the BRDF of the base layer is replaced (or partially replaced) by the same kind of lobe pointed back along the light vector instead of the reflection vector.

Sample model: https://github.com/KhronosGroup/glTF-Sample-Assets/pull/285

If any technical implementation details need further clarification, please ask @mklefrancois or use this issue.

/cc @jim-ec @abwood

Start by reading the KHR_materials_retroreflection draft in KhronosGroup/glTF pull request 2610, then locate the Sample Renderer's PBR shader. Use the sample model from glTF-Sample-Assets pull request 285 to verify that the extension renders as intended, with the retroreflection lobe affecting the base layer.

Written by the indexing model from the issue text.