To clarify a few points:
- it can't be done as an extension, the internals of sf::Font have to be changed to implement this feature
- pixelated fonts already work perfectly in most cases, thanks the latest changes in SFML 2.2; this case is special because the text is stretched
- if this feature was implemented, it would also work fine on fonts that are not natively pixelated
- it has to be an additional argument to the load functions of sf::Font, it can't be changed after the glyphs are rasterized