Specify the size for the map's Loader item

This is currently required by the new runtime.

kria-cluster-3d-demo/qml/main.qml

@@ -154,6 +154,8 @@ Item {
                             visible: active
                             source: ""
                             asynchronous: true
+                            width: 2560
+                            height: 960
                         }
                     }
                 ]