GLTFLoader: Node does not support multiple references

[donmccurdy]

Per KhronosGroup/glTF#1433, a single node can reference up to 1 mesh, 1 camera, AND 1 light, not OR. Our current implementation picks one and ignores the others. So if a node references a camera and a light, we should just turn it into a THREE.Group. Also note from that thread that a node referencing a mesh could also be a joint, in which case we need to output Bone > Mesh.

Example: https://github.com/KhronosGroup/glTF/files/2317078/mesh-camera.zip