iconGrid: Avoid animating the same icon twice

If the icon proper has opacity of zero then that's probably because a
clone of it is animating. So avoid animating the source actor too.

And if there's any other reason for the opacity being zero, still don't
animate it because we can't see it :)

Closes: https://gitlab.gnome.org/GNOME/gnome-shell/issues/2167

js/ui/iconGrid.js

@@ -195,7 +195,7 @@ function zoomOutActorAtPos(actor, x, y) {
 }
 
 function animateIconPosition(icon, box, flags, nChangedIcons) {
-    if (!icon.has_allocation() || icon.allocation.equal(box)) {
+    if (!icon.has_allocation() || icon.allocation.equal(box) || icon.opacity === 0) {
         icon.allocate(box, flags);
         return false;
     }