Reposition outputs after potentially changing mode

Currently outputs aren't repositioned again after a mode change, which
can cause overlaps.

src/niri.rs

@@ -722,10 +722,10 @@ impl State {
                 self.niri.output_resized(output);
             }
 
-            self.niri.reposition_outputs(None);
-
             self.backend.on_output_config_changed(&mut self.niri);
 
+            self.niri.reposition_outputs(None);
+
             if let Some(touch) = self.niri.seat.get_touch() {
                 touch.cancel(self);
             }