Exposes inner backends to allow extra functionalities if the user needs them.

[Nazariglez]

Allowing access to the inner backends will allow the user to use specific features that the backend have but there is no interface to use it from notan.

This is consider an advance feature, using the inner backend directly can break the application or corrupt states (ie: graphics backend can be easily corrupter using OpenGL directly).

It will be probably a breaking change, so this will be included eventually in a branch marked as breaking, never in a minor/small update of the lib.

[Nazariglez]

Closing, this will be addressed by #299. We just fully embrace one backend and we can give access to the user easily for that case.

We don't need this issue anymore.