Utility loaders to asynchronously create texture at launch and manage it correctly

Another proposal for #2049 which encapsulate Texture derived class inside loader to avoid confusion.
Those derived classes are only usefull when registering loaders and generates no conflict with TextureLoader...

What do you think ?
I understand what you are trying to do here, but this can fail horribly for other users depending on the registered loaders. Also, TextureData is not guaranteed to return a Pixmap in consumePixmap, especially if you don't call prepare first.