magnifier: Update zoom region right after enabling the magnifier

We want the zoom region to always be where the mouse cursor is, so make
sure to update it initially, too.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2234>

js/ui/magnifier.js

@@ -215,6 +215,8 @@ var Magnifier = class Magnifier {
         if (!this._pointerWatch) {
             let interval = 1000 / 60;
             this._pointerWatch = PointerWatcher.getPointerWatcher().addWatch(interval, this.scrollToMousePos.bind(this));
+
+            this.scrollToMousePos();
         }
     }