iconGrid: avoid recreating icons unnecessarily

Add a short-circuit when the size doesn't change. Taken from part
of 26580f8f2c (Giovanni Campagna). This greatly reduces the reentrancy
of recreating icons that seems to be responsible for many crashers,
though the exact mechanism for the crashes isn't clear. See:

https://bugzilla.gnome.org/show_bug.cgi?id=648450
https://bugzilla.redhat.com/show_bug.cgi?id=707652
https://bugzilla.redhat.com/show_bug.cgi?id=781780

js/ui/iconGrid.js

@@ -147,6 +147,11 @@ BaseIcon.prototype = {
             size = found ? len : ICON_SIZE;
         }
 
+        // don't create icons unnecessarily
+        if (size == this.iconSize &&
+            this._iconBin.child)
+            return;
+
         this._createIconTexture(size);
     }
 };