KHR_materials_cmnConstant

[donmccurdy]

Based on the syntax of #1075. For mobile devices trying to maintain 90+ FPS, I think it may be worth keeping an unlit shading model available.

[donmccurdy]

Hm as @zellski pointed out off thread, emissiveFactor and emissiveTexture are already in the default PBR material spec, so there's nothing new here. We can set an emissive and zero out the other factors.

If an exporter specifies that something should be unlit by zeroing-out the other factors, we expect the importer to check each factor and assign the simpler shading model? I thought we'd intended for exporters to be able to provide both metal/rough and spec/gloss and let the runtime choose which to use, but this leaves us no way of specifying whether unlit shading is explicitly a fallback or not. Maybe that's OK? I'm not sure.

[donmccurdy]

On second thought, please ignore the content of this PR for now. I will follow up on #1075.

Updated this PR, it is ready for review and feedback.

[zellski]

Perhaps open this back up, in empty/boolean form, given recent developments in #1075?

[donmccurdy]

Sounds good. I've updated this as an "empty" extension, and it should be ready for review or feedback.

[donmccurdy]

Closing this; going to work toward including this along with blinnPhong as a single extension. See #1111.