KHR_material_emissive_strength: missing extension support

[bhouston]

I believe that both basic emissive as well as the newer KHR_materials_emissive_strength extension are not support supported in this project?

I mention that because looking at the render results of khronos-EmissiveStrengthTest (KHR_materials_emissive_strength) seem to suggest that.

In Stellar:

In Three.js:

In three-gpu-pathtracer:

In the current V-Ray, no emission at all:

[jasondavies]

I'm going to work on this.