macOS: Fix crash after close a fullscreen window.
This crash only happens when user closes a fullscreen window. GTK finalized it's impl object GdkWindowImplQuartz after receiving windowWillClose: . But the GdkQuartzNSWindow is not deleted yet at that time, it will receive setStyleMask: after windowWillClose.
In that case, GTK will call _gdk_quartz_window_update_fullscreen_state() and access finalized GdkWindowImplQuartz object and cause crash.