Decouple public API from glutin and glium

[vadixidav]

In several places, elements of glutin and glium are publicly exposed by the API. From the user's perspective, only necessary entry points should be exposed and wrapped by structs/implement traits which are in the library and then hide everything else about the implementation with the backend.

[vadixidav]

On example of this currently is that the texture type being used now is an Rc, where Texture2d comes from glium.