Use inheritance to implement wrapper struct types

[ChrisThrasher]

We can save code and remove the ugly This parameter if we instead inherit from sf:: types.

Here's an example: master...simplify_sf_buffer

[eXpl0it3r]

Does this really work, i.e. is that type not part of the public C interface?

[ChrisThrasher]

The C interface doesn’t know anything about how the type is defined. That’s why we’re allowed to do this.

[ChrisThrasher]

master...remove_This

I've started implementing this and am liking how it looks. I still think this is something we should pursue.

For a select few types like sfShader we'll need to add some constructors that delegate to the underlying sf::Shader constructors but for most types we can rely on aggregate initialization so no more code is required.