[glTF] Support materials without pbrMetallicRoughness

[footballhead]

GltfMaterialSelector::DetermineMaterial will return the error material for materials that are not unlit and also lack pbrMetallicRoughness.

However, the spec says that the defaults should be applied for pbrMetallicRoughness if not present (see 5.19). That means that DetermineMaterial should still return PPX_MATERIAL_IDENT_STANDARD and the loading code should apply defaults for the missing values.

This would allow BigWheels to correctly render models like:

• AnisotropyStrengthTest