Optimize parameter passing to avoid unnecessary copying

src/world/LevelEvents.cpp

@@ -3,14 +3,14 @@
 
 using std::vector;
 
-void LevelEvents::listen(lvl_event_type type, chunk_event_func func) {
+void LevelEvents::listen(lvl_event_type type, const chunk_event_func& func) {
 	auto& callbacks = chunk_callbacks[type];
 	callbacks.push_back(func);
 }
 
 void LevelEvents::trigger(lvl_event_type type, Chunk* chunk) {
 	const auto& callbacks = chunk_callbacks[type];
-	for (chunk_event_func func : callbacks) {
+	for (const chunk_event_func& func : callbacks) {
 		func(type, chunk);
 	}
 }