background: rebuild background, not just animation on resume

Previously, we would only refresh the animation on resume
(to handle clock skew).

But we actually need to rebuild the background, too, on nvidia,
so we should just do a full background change.

js/ui/background.js

@@ -264,7 +264,7 @@ var Background = new Lang.Class({
             (lm, aboutToSuspend) => {
                 if (aboutToSuspend)
                     return;
-                this._refreshAnimation();
+                this.emit('changed');
             });
 
         this._settingsChangedSignalId = this._settings.connect('changed', () => {