backends: Prefer a scale closest to the DPI target on the upper side

Right now, we pick the strictly closest scale to the target DPI, which may err on a bigger scales (and smaller screen space) by very close. E.g. a 13.3" 3840x2400 screen will prefer 3x (i.e. virtually 1280x800) over 2x), the former being 22DPI under the 135 target, the latter 33 over.

It is more usual that users prefer more screen space over fuller pixels, especially in laptop form factor, so err on the screen space side and pick the scale that matches the DPI target best on the upper side only. Users actually preferring the bigger scale in similar scenarios can still configure it so.