gtk_init: Fix debug flags handling when a display is already open

In PyGObject gdk_init() is called before gtk_init() and thus there is
already a default display open when gtk_init() is called.
The code assigning the display to the debug_flags struct gets only
called when the default display changes, which never happens
when there already is one. As a result GTK_DEBUG=interactive
doesn't do anyting with Python apps.

This makes it call the change callback in case a display is already
there.

See https://gitlab.gnome.org/GNOME/pygobject/issues/166

gtk/gtkmain.c

@@ -762,7 +762,7 @@ G_GNUC_END_IGNORE_DEPRECATIONS
 
   display_manager = gdk_display_manager_get ();
   if (gdk_display_manager_get_default_display (display_manager) != NULL)
-    _gtk_accessibility_init ();
+    default_display_notify_cb (display_manager);
 
   g_signal_connect (display_manager, "notify::default-display",
                     G_CALLBACK (default_display_notify_cb),