ezoom: Add option to choose between no smoothing and linear smoothing

Hello,

Currently, ezoom always uses linear interpolation when zoomed in. When using quite strong zooms (e.g. 10x), this leads to very blurry effects, which is not necessarily so good-looking actually, compared to just getting big pixels, and might even bring headaches (because the brain of the user thinks the user has troubles with eyes, and tries to focus more).

So I'd say the user should be able to use between linear interpolation and no interpolation at all. I will submit a patch for this as a pull request.

This will also pave the way for being able to choose between various algorithms: linear, cubic, sinc, or no interpolation, which I will work on later. Preliminary experiments have shown that the obtained results are diverse, so it's better to just let the user choose what she ends up preferring.

Samuel