wayland: calculate screen physical size
wayland: calculate screen physical size

A simple calculation is done so far (assuming monitor areas never overlap)
so gdk_screen_get_width/height_mm return meaningful values.

https://bugzilla.gnome.org/show_bug.cgi?id=753621

alexlarsson/gtk,jigpu/gtk,chergert/gtk,jadahl/gtk,chergert/gtk,jigpu/gtk,davidgumberg/gtk,jadahl/gtk,grubersjoe/adwaita,jadahl/gtk,chergert/gtk,jadahl/gtk,alexlarsson/gtk,alexlarsson/gtk,chergert/gtk,jigpu/gtk,alexlarsson/gtk,jigpu/gtk,chergert/gtk,chergert/gtk,davidgumberg/gtk,grubersjoe/adwaita,chergert/gtk,davidgumberg/gtk,chergert/gtk,jadahl/gtk,alexlarsson/gtk,jigpu/gtk,jigpu/gtk,jadahl/gtk,jigpu/gtk,davidgumberg/gtk,grubersjoe/adwaita,grubersjoe/adwaita,davidgumberg/gtk,davidgumberg/gtk,jadahl/gtk,davidgumberg/gtk,jadahl/gtk,alexlarsson/gtk,alexlarsson/gtk,jigpu/gtk,grubersjoe/adwaita,alexlarsson/gtk,grubersjoe/adwaita,davidgumberg/gtk,grubersjoe/adwaita,grubersjoe/adwaita