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
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