gint gdk_screen_get_monitor_scale_factor ( GdkScreen* screen, gint monitor_num )
Returns the internal scale factor that maps from monitor coordinates to the actual device pixels. On traditional systems this is 1, but on very high density outputs this can be a higher value (often 2).
This can be used if you want to create pixel based data for a
particular monitor, but most of the time you’re drawing to a window
where it is better to use