add g_return_if_fail for monitor number <= 0

add g_return_if_fail for monitor number <= 0

2002-10-16  Havoc Pennington  <hp@redhat.com>

	* gdk/x11/gdkscreen-x11.c (gdk_screen_get_monitor_geometry): add
	g_return_if_fail for monitor number <= 0

	* gtk/gtkmenu.c (gtk_menu_position): handle case where pointer
	is outside the screen.

	* gtk/gtkwindow.c (get_monitor_center): factor out
	the code to position a window in center
	(gtk_window_compute_configure_request): use here
	(gtk_window_constrain_position): use here, the
	code here wasn't multihead-aware

	* gdk/x11/gdkevents-x11.c (get_real_window): function to
	return the window the event describes, rather than the window
	that received the event.
	(gdk_event_translate): use result of that function for
	filling in GdkEvent::window, which only matters for
	MapNotify/UnmapNotify and ConfigureNotify, in practice.

	Unbreaks various things using SubstructureNotifyMask.