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.
They won't fail with OSError or CalledProcessError though.  I was writing this suggestion while thinking about refactoring the whole thing in a secondary helper, see latest commit.