CompressedTextureLoader: Refactor parsing.

[Mugen87]

The idea is to remove _parser and just let concrete instances of CompressedTextureLoader implement Loader.parse(). In this way, DDSLoader, KTXLoader and PVRLoader become more similar to the rest of the loaders.

If approved, I'm going to change DataTextureLoader, too.

[mrdoob]

Sounds good to me 👌

[mrdoob]

Thanks!