Handle the full range of glTF materials

[kring]

Currently the base material used for glTF is pretty simple:

(If you don't see GltfMaterial in the Content Browser in the Unreal Editor, click View Options -> Show Plugin Content.)

This is just a subset of the what a glTF material can do. It also requires the material to have a texture, which is not ok.

So this issue is to flesh out the base material - or create multiple base materials if necessary - in order to support the full set of glTF material options.

[kring]

I posted a question related to this to UDN:
https://udn.unrealengine.com/questions/588199/set-static-switch-parameter-on-runtime-created-uma.html