https://bugzilla.gnome.org/show_bug.cgi?id=654656

Clutter may be used together with GTK+, which indirectly may use
XInput2 too, so the cookie data must persist when both are handling
events.

What happens now in a nutshell is, Clutter is only guaranteed to allocate
the cookie itself after XNextEvent(), and only frees the cookie if its
XGetEventData() call allocated the cookie data.

The X[Get|Free]EventData() calls happen now in clutter-event-x11.c as
hypothetically different event translators could also handle other set
of X Generic Events, or other libraries handling events for that matter.