Fix reference-counting in set_event_filter

This previously resulted in the following test failure with Perl 5.38.2
on Debian unstable (https://bugs.debian.org/1064761):

    t/core_events.t ................. 1/? Can't use an undefined value as a subroutine reference during global destruction.
    t/core_events.t ................. Dubious, test returned 22 (wstat 5632, 0x1600)
    All 691 subtests passed
            (less 1 skipped subtest: 690 okay)

Author: cjwatson

src/Core/Events.xs

@@ -102,7 +102,8 @@ void
 events_set_event_filter(callback)
 	SV* callback
 	CODE:
-		eventfiltersv = callback;
+		SvREFCNT_dec(eventfiltersv);
+		eventfiltersv = SvREFCNT_inc_simple(callback);
 		SDL_SetEventFilter((SDL_EventFilter) eventfilter_cb);
 
 AV *