[ft] Debug why fonts are displayed too small?

[Felix E. Klee]

After a recent Arch update, fonts at least in GTK+ 3 applications are
rendered too small. They appear as if they are rendered at 96 DPI, while
the display has a much higher resolution:

    $ xrdb -query | grep dpi
    xft.dpi:        192
    $ xdpyinfo | grep -B2 resolution
    screen #0:
      dimensions:    2880x1620 pixels (381x214 millimeters)
      resolution:    192x192 dots per inch
    $ xrandr | grep mm
    eDP1 connected 2880x1620+0+0 (normal left inverted right x axis y
    axis) 340mm x 190mm

GTK+ 3 uses Freetype for rendering fonts. How do I find out what has
changed / what is wrong?b