Select popups not scaled when HiDPI scaled output is offset

Select popups aren't scaled correctly if the current output is both scaled (2x) and offset from the origin. This doesn't happen if the display is at pos 0,0.

Happens with Falkon and Qutebrowser in wlroots-based wayland compositors (and possibly others, haven't tested)

Original report with screenshots: ifreund/river#195
Downstream: Qutebrowser#6043