KHR_material_specular: missing extension support

[bhouston]

There is an additional extension called KHR_materials_specular that is not currently supported.

Here is some reference images using this model khronos-SpecularTest.

Three.js render:

Current V-Ray render:

[jasondavies]

I'm going to work on this.

[jasondavies]

I managed to get some of the way there, but for some reason color factor > 1.0 doesn't work. I wonder if we need to use a falloff texture to implement this properly?