background: Mark pattern backgrounds as loaded

Otherwise we don't let GNOME Shell startup to proceed. Noticed
by accidentally running on the memory GSettings backend.

https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1381

js/ui/background.js

@@ -347,6 +347,8 @@ var Background = GObject.registerClass({
             this.set_color(color);
         else
             this.set_gradient(shadingType, color, secondColor);
+
+        this._setLoaded();
     }
 
     _watchFile(file) {