widget: Skip popovers in allocation

Native widgets get allocated via their surface,
so can skip them here. This avoids criticals when
re-mapping a popover for the second time, as can
be seen e.g. in the 'Selections' demo in gtk4-demo.

gtk/gtkwidget.c

@@ -3872,7 +3872,8 @@ gtk_widget_ensure_allocate_on_children (GtkWidget *widget)
        child != NULL;
        child = _gtk_widget_get_next_sibling (child))
     {
-      gtk_widget_ensure_allocate (child);
+      if (gtk_widget_should_layout (child))
+        gtk_widget_ensure_allocate (child);
     }
 }