[scene] Swtich from - to ~ for entity id index

It saves only 1 bit in the set, but it could save on allocations. Also
takes care of a FIXME :P

include/QF/scene/entity.h

@@ -114,7 +114,7 @@ ENTINLINE
 void
 EntQueue_AddEntity (entqueue_t *queue, entity_t *ent, int queue_num)
 {
-	int         id = -ent->id;//FIXME use ~
+	int         id = ~ent->id;
 	if (!set_is_member (queue->queued_ents, id)) {
 		// entity ids are negative (ones-complement)
 		set_add (queue->queued_ents, id);