Merge: Small clean up and improvements to MNit from WBTW

Merge: Small clean up and improvements to MNit from WBTW

The new OpenGL parameters affect the display of images for a better pixelated look:
* When zoomed out, the image is smoother, the color values are interpolated from the nearest source pixels.
* When zoomed in, the image is displayed pixelated.

You may recognize the `Int::next_pow` method, `inkscape_tools` use something similar for the same purpose. Actually, with this PR, `inkscape_tools` may not need it anymore and produce smaller PNG files.

Pull-Request: #1284
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Romain Chanoir <chanoir.romain@courrier.uqam.ca>