Many applications use Xft.dpi to scale their contents (e.g. Chromium,
kitty, alacritty...). This value also gets set by DE setting managers
and can be manually set in ~/.Xresources if using, e.g., i3.
This should make HiDPI on Linux more consistent even when not using
GTK-specific methods (e.g. GDK_SCALE=2).
Note that we still consider GTK scaling, so it's possible to use the two
independently.
Closes#1243
ad503b8c4f