scaling: Improve scaling

Issues fixed by this change:

- Empty space above and below layouts.
- The compact shape, which is supposed to be used on small screens,
  did not scale up until it reached the border of the screen, on some
  screens, resulting in empty space next to it, and smaller buttons.
- The compact shape was used on some screens in horizontal orientation,
  instead of the wide shape.
- The panel was rather small on large screens.

Part-of: <https://gitlab.gnome.org/World/Phosh/squeekboard/-/merge_requests/677>

src/outputs.rs

@@ -322,7 +322,7 @@ pub struct Mode {
     pub height: i32,
 }
 
-#[derive(Clone, Copy, Debug)]
+#[derive(Clone, Copy, Debug, PartialEq, PartialOrd, Eq, Ord)]
 pub struct Millimeter(pub i32);
 
 impl DivCeil<i32> for Millimeter {