Fix GDNative compat breakage due to dangling Variants fix

This moves the instance id member from Variant to the ObjectRC so that Variant is still the same size as before the fix (and also regardless if debug or release build).

core/object_rc.h

@@ -46,6 +46,11 @@ class ObjectRC {
 	std::atomic<uint32_t> _users;
 
 public:
+	// This is for allowing debug builds to check for instance ID validity,
+	// so warnings are shown in debug builds when a stray Variant (one pointing
+	// to a released Object) would have happened.
+	const ObjectID instance_id;
+
 	_FORCE_INLINE_ void increment() {
 		_users.fetch_add(1, std::memory_order_relaxed);
 	}
@@ -63,7 +68,8 @@ public:
 		return _ptr.load(std::memory_order_acquire);
 	}
 
-	_FORCE_INLINE_ ObjectRC(Object *p_object) {
+	_FORCE_INLINE_ ObjectRC(Object *p_object) :
+			instance_id(p_object->get_instance_id()) {
 		// 1 (the Object) + 1 (the first user)
 		_users.store(2, std::memory_order_relaxed);
 		_ptr.store(p_object, std::memory_order_release);