GLTFLoaderBase shouldn't be disposable

[mgsx-dev]

It may be "disposed" when an error occurring in order to prevent memory leaks. Some game/application may want to try to load a model and handle exception without crashing.

However it could be misleading: some people might think that they should dispose the loader. It shouldn't implement Disposable interface and dispose method should be protected and renamed as "cleanup" instead.

[prineside]

Totally agree. Also have spent too much time trying to re-use the loaders (and they have managed to load 50 out of 217 GLTF 2.0 sample models somehow) because Disposable is usually used for something heavy / reusable such as Texture / Batch in LibGDX.