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: https://github.com/SFML/CSFML/compare/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.