GtkWidget: Ensure the style context always has an screen.

Things like font settings depend on the screen, and widgets
like GtkTextView trigger queries on widgets without screen
when the parent window is being destroyed.

gtk/gtkwidget.c

@@ -8388,7 +8388,7 @@ do_screen_change (GtkWidget *widget,
       _gtk_tooltip_hide (widget);
 
       context = gtk_widget_get_style_context (widget);
-      gtk_style_context_set_screen (context, new_screen);
+      gtk_style_context_set_screen (context, gtk_widget_get_screen (widget));
 
       g_signal_emit (widget, widget_signals[SCREEN_CHANGED], 0, old_screen);
     }