Revert "js/ui: Use Clutter.OffscreenRedirect.ON_IDLE"

This reverts commit c0c027c608. Because for
some reason animating external opacity and position is still incurring
internal repaints, which disables offscreening and makes fading of
overlapping actors look wrong. `ON_IDLE` should be fixed in mutter before
it is used (in boxpointer at least) again.

Fixes https://gitlab.gnome.org/GNOME/gnome-shell/issues/2270

js/ui/panel.js

@@ -794,7 +794,7 @@ class Panel extends St.Widget {
         super._init({ name: 'panel',
                       reactive: true });
 
-        this.set_offscreen_redirect(Clutter.OffscreenRedirect.ON_IDLE);
+        this.set_offscreen_redirect(Clutter.OffscreenRedirect.ALWAYS);
 
         this._sessionStyle = null;