GdkWindow's before_process_all_updates() and after_process_all_updates()
wrongly assume that all displays are from the same class, which is not
the case if for example a client open different displays with different
backends such as X11 and Wayland.

Use the actual class for each display in the display list to avoid a
crash when mixing displays from different classes.

Fix suggested by Christian Persch <chpe@gnome.org> in bug #776472.

https://bugzilla.gnome.org/show_bug.cgi?id=776472