Fix multiple issues in WorkerThreadPool

- Fix project settings being ignored.
- Made usages of `native_thread_allocator` thread-safe.
- Remove redundant thread-safety from `low_priority_threads_used`, `exit_threads`.
- Fix deadlock due to unintended extra lock of `task_mutex`.

main/main.cpp

@@ -1454,6 +1454,19 @@ Error Main::setup(const char *execpath, int argc, char *argv[], bool p_second_ph
 #endif
 	}
 
+	// Initialize WorkerThreadPool.
+	{
+		int worker_threads = GLOBAL_GET("threading/worker_pool/max_threads");
+		bool low_priority_use_system_threads = GLOBAL_GET("threading/worker_pool/use_system_threads_for_low_priority_tasks");
+		float low_property_ratio = GLOBAL_GET("threading/worker_pool/low_priority_thread_ratio");
+
+		if (editor || project_manager) {
+			WorkerThreadPool::get_singleton()->init();
+		} else {
+			WorkerThreadPool::get_singleton()->init(worker_threads, low_priority_use_system_threads, low_property_ratio);
+		}
+	}
+
 	// Initialize user data dir.
 	OS::get_singleton()->ensure_user_data_dir();