[glTF] Support materials without pbrMetallicRoughness

`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](https://registry.khronos.org/glTF/specs/2.0/glTF-2.0.html#reference-material)). 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](https://github.com/KhronosGroup/glTF-Sample-Assets/tree/main/Models/AnisotropyStrengthTest)