Currently we have support for PBR materials, but only using the SpecularRoughness models.  In addition we should support the MetalRoughness model.  This could either be done via a CustomMaterial, extending the DefaultMaterial, or by adding an additional default material generator for MetalRoughness.  Some experimentation has been done so far.

The plan is to implement a MetalRoughness  based off of the Principled BSDF node in the Blender Eevee renderer.  This should get us where we want to be with regard to PBR support and GLTF2 import.