GdkDisplayX11: Don't use substructure events in internal accounting

GdkDisplayX11: Don't use substructure events in internal accounting

We may receive events because SubstructureNotifyMask has been selected
for the root window. (Most likely, this would occur because GTK+
is being used inside a window manager like Metacity or Mutter.)
This can confuse various types of internal accounting, so detect
such events and comprehensively ignore them for GDK's internal
purposes. We still need to generate GDK events for these cases
because you can select for substructure events with
GDK_SUBSTRUCTURE_MASK.

https://bugzilla.gnome.org/show_bug.cgi?id=685460