Add support for KHR_material_cmnBlinnPhong extension

[Kupoman]

There is a discussion on the extension here: KhronosGroup/glTF#947
And a pull request here: KhronosGroup/glTF#1075

[Kupoman]

Removing from version 1.1 targets since the extension is not yet complete.

[Moguri]

Clearing the milestone on this until the spec is more stable/finalized.

[donmccurdy]

An update — KHR_materials_common and its proposed alternative, KHR_materials_cmnBlinnPhong, will not be created for glTF 2.0 at this time.

The strongest feedback from the community was around shadeless materials, so we are moving forward with the simpler KHR_materials_unlit:

• Specification
• Demo viewer w/ patched support

The extension is not yet ratified, but Sketchfab and Facebook will begin using it shortly and the schema is unlikely to change — I think it would be safe to include with the next milestone here, and I have also proposed it for the Khronos exporter.

[Kupoman]

Closing this issue since this extension is unlikely to go anywhere. Thanks for the update @donmccurdy.