arctica-greeter-check-hidpi: Fix HiDPI auto-detection.

 When calculating the rectangle height, using `get_monitor_geometry`
 returns "application pixels", rather than "device pixels". We need to
 multiply by the scale factor to convert to the correct value.

 https://valadoc.org/gdk-3.0/Gdk.Screen.get_monitor_geometry.html

 Ported from slick-greeter by Mike Gabriel <mike.gabriel@das-netzwerkteam.de>

arctica-greeter-check-hidpi

@@ -44,7 +44,7 @@ def get_window_scale():
             or (width_mm == 16 and height_mm == 10)):
             return 1
 
-        if rect.height < 1500:
+        if rect.height * monitor_scale < 1500:
             return 1
 
         if width_mm > 0 and height_mm > 0: