altTab: Ensure style of this._list before calculating icon sizes

We're calculating icon sizes for the alt tab switcher early and at a
point where the style attributes of this._list are not loaded yet. To
make sure the value of this._list.spacing is correct, call
ensure_style() on this._list before accessing the spacing.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/587

js/ui/altTab.js

@@ -728,6 +728,7 @@ class AppSwitcher extends SwitcherPopup.SwitcherList {
                 j++;
         }
         let themeNode = this._items[j].get_theme_node();
+        this._list.ensure_style();
 
         let iconPadding = themeNode.get_horizontal_padding();
         let iconBorder = themeNode.get_border_width(St.Side.LEFT) + themeNode.get_border_width(St.Side.RIGHT);