Scale the window by the assumed DPI scaling factor, using 96 DPI as
base. For example, a screen that reports 192 DPI is assumed to have a
DPI scale factor 2. The window will then be created with twice the size.
For robustness reasons, we accept only integer DPI scales between 1 and
9. We also error out if the X and Y scales are very different, as this
most likely indicates a multiscreen system with botched size reporting.
I'm not sure if reading the X server's DPI is such a good idea - maybe
the Xrdb "Xft.dpi" value should be used instead. The current method
follows what xdpyinfo does.
This can be disabled with --hidpi-window-scale=no.