Add protections against registering classes that didn't use GDCLASS()

(cherry picked from commit a61cdc8860)

include/godot_cpp/core/class_db.hpp

@@ -173,6 +173,7 @@ public:
 
 template <class T, bool is_abstract>
 void ClassDB::_register_class(bool p_virtual) {
+	static_assert(TypesAreSame<typename T::self_type, T>::value, "Class not declared properly, please use GDCLASS.");
 	instance_binding_callbacks[T::get_class_static()] = &T::_gde_binding_callbacks;
 
 	// Register this class within our plugin