Montage: previews don't scale up on HiDPI screen in KDE, spill beyond display borders

[NHOrus]

In montage mode, in KDE with scale level 2, previews are still at original resolution from config file, not upscaled for best experience.

Also, I think montage width/length are calculated wrong: Around 3/4th of all previews are invisible, located off-screen, so if I start selecting image with arrow keys, selection disappears, and previews aren't scrolled up when I move selection down, also offscreen, for some time.