popupMenu: Fix popup menu layouts in RTL directions

The math for inverting RTL layouts (which was copy/pasted from
ClutterBoxLayout) was incorrect for non-zero child offsets.

https://bugzilla.gnome.org/show_bug.cgi?id=704542

js/ui/popupMenu.js

@@ -317,7 +317,7 @@ const PopupBaseMenuItem = new Lang.Class({
             }
 
             if (direction == Clutter.TextDirection.RTL)
-                childBox.x1 = width - x - availWidth;
+                childBox.x1 = box.x1 + (box.x2 - x - availWidth);
             else
                 childBox.x1 = x;
             childBox.x2 = childBox.x1 + availWidth;