[glTF] Calculate normals if not provided

[footballhead]

GltfLoader::LoadMeshData will provide (0,0,0) for normals if not present. However 3.7.2.1 says:

When normals are not specified, client implementations MUST calculate flat normals and the provided tangents (if present) MUST be ignored.

This would allow BigWheels to correctly render:

• SimpleMaterial