This adds an optional screen argument to get_dpi/apply_dpi and set_dpi
to store DPI values per screen.
This can be used in your config to manage different DPI values for an
internal and external display:
beautiful.xresources.set_dpi(125, 1)
beautiful.xresources.set_dpi(94, 2)
This is meant to be the foundation for more evolved DPI handling in
awesome.
Closes https://github.com/awesomeWM/awesome/pull/336.