popupMenu: round spacing to an integer

StThemeNode.get_length() doesn't necessarily return an integer pixel
value, and our code produces non-integer positions in that case. So
round the spacing.

https://bugzilla.gnome.org/show_bug.cgi?id=645647

js/ui/popupMenu.js

@@ -61,7 +61,7 @@ PopupBaseMenuItem.prototype = {
     },
 
     _onStyleChanged: function (actor) {
-        this._spacing = actor.get_theme_node().get_length('spacing');
+        this._spacing = Math.round(actor.get_theme_node().get_length('spacing'));
     },
 
     _onButtonReleaseEvent: function (actor, event) {