combobox: allocate the right border to the arrow and not to the label

When the combobox is in menu mode, the right padding of the togglebutton
inside, should be allocated to its rightmost children, which is the
arrow.

https://bugzilla.gnome.org/show_bug.cgi?id=646882

gtk/gtkcombobox.c

@@ -2449,7 +2449,7 @@ gtk_combo_box_size_allocate (GtkWidget     *widget,
           gtk_widget_get_preferred_size (priv->arrow, &req, NULL);
           child.width = req.width;
           if (!is_rtl)
-            child.x += width - req.width;
+            child.x += width - req.width - button_padding.right;
           child.width = MAX (1, child.width);
           child.height = MAX (1, child.height);
           gtk_widget_size_allocate (priv->arrow, &child);
@@ -2467,7 +2467,7 @@ gtk_combo_box_size_allocate (GtkWidget     *widget,
             {
               child.x += req.width;
               child.width = allocation->x + allocation->width
-                - (border_width + button_border.right + focus_width + focus_pad)
+                - (border_width + focus_width + focus_pad)
                 - child.x;
             }
           else