Adding full support for custom materials to GltfLoader

Good point. In fact, this is true for the whole GltfLoader and all of its subcomponents in general. You made me realize that there is no good way to access the GltfLoader for configuration. This makes things rather difficult for what I want to achieve.

Maybe we could declare the internal list of GltfMaterialFactorys in the GltfLoader as protected. With this, one could simply extend GltfLoader and set up this list directly (edit: or better by using apropriate add and remove methods) in the constructor. I don’t like this approach, but I currently have no better idea.

Btw, GltfLoader already has some public configuration methods (e.g. registerExtension(...) and registerDefaultExtrasLoader(...)) that are practically inaccessible.