Fallback on xrandr to get linux DPI info.

xdpyinfo is not always available on Linux (even with a DE), whereas
xrandr is (AFAICT) at least dependency of KDE and XFCE.  But xrandr
fails when used remotely, so keep it too.
Yes, I can run KDE and whatever programs I usually use perfectly fine without `xdpyinfo`.
Checking for `command -v xdpyinfo` may work there but for xrandr that is not the problem; rather, in certain cases (e.g. when run under VNC), `xrandr` will be present but unable to report the resolution.
I would also argue that going first through `command -v` is a violation of [EAFP](https://docs.python.org/3/glossary.html#term-eafp)...