niri: only emit displayScales when a scale changed

ref #3842
port 2.0

quickshell/Services/NiriService.qml

@@ -334,9 +334,18 @@ Singleton {
             }
         }
 
+        if (sameScales(displayScales, scales))
+            return;
         displayScales = scales;
     }
 
+    function sameScales(a, b) {
+        const names = Object.keys(a);
+        if (names.length !== Object.keys(b).length)
+            return false;
+        return names.every(name => a[name] === b[name]);
+    }
+
     function sortWindowsByLayout(windowList) {
         const enriched = windowList.map(w => {
             const ws = workspaces[w.workspace_id];