appDisplay: Extend faded edges beyond fade area

Now that the fade effect was reverted to the old behavior by default,
the app grid must opt-in extend faded edges beyond the regular fade
area.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1831>

js/ui/appDisplay.js

@@ -364,6 +364,9 @@ var BaseAppView = GObject.registerClass({
         }
 
         this._scrollView.update_fade_effect(fadeMargin);
+        const effect = this._scrollView.get_effect('fade');
+        if (effect)
+            effect.extend_fade_area = true;
     }
 
     _updateFade() {