st: Round CSS units to the nearest multiple of the scale factor
Actors themed through CSS should ideally get sizes and positions that conform to the "pixel grid". A notorious example is the panel that has a height of 1.86em. On unchanged font settings and hidpi that translates to 55px, which leaves the workarea with "half pixels" that hidpi wayland applications don't know how to fully cover.
If the requested height is a multiple of the scale factor, the workarea and maximized applications can then work on full pixels.
Closes: #91 (closed)