ResourceLoader: Fixup management of thread-specific status

- Allows the message queue override to flush after loading each resource, which was the original intent.
- Removes a redundant call to mark the thread as safe-for-nodes.

core/io/resource_loader.cpp

@@ -304,9 +304,10 @@ void ResourceLoader::_thread_load_function(void *p_userdata) {
 	thread_load_mutex.unlock();
 
 	// Thread-safe either if it's the current thread or a brand new one.
-	bool mq_override_present = false;
+	thread_local bool mq_override_present = false;
 	CallQueue *own_mq_override = nullptr;
 	if (load_nesting == 0) {
+		mq_override_present = false;
 		load_paths_stack = memnew(Vector<String>);
 
 		if (!load_task.dependent_path.is_empty()) {
@@ -326,10 +327,6 @@ void ResourceLoader::_thread_load_function(void *p_userdata) {
 	}
 	// --
 
-	if (!Thread::is_main_thread()) {
-		set_current_thread_safe_for_nodes(true);
-	}
-
 	Ref<Resource> res = _load(load_task.remapped_path, load_task.remapped_path != load_task.local_path ? load_task.local_path : String(), load_task.type_hint, load_task.cache_mode, &load_task.error, load_task.use_sub_threads, &load_task.progress);
 	if (mq_override_present) {
 		MessageQueue::get_singleton()->flush();