Walk the shadow passes and the culled model groups on worker threads in the Vulkan renderer

rendering/vulkan14/vulkan14tasks.h

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+/*
+This Source Code Form is subject to the
+terms of the Mozilla Public License, v.
+2.0. If a copy of the MPL was not
+distributed with this file, You can
+obtain one at
+http://mozilla.org/MPL/2.0/.
+*/
+
+// the background threads of the vulkan renderer (the stage M5 of the plan): the work which would hold up a frame, done while the
+// frames go on. the pipelines of the scene and the textures made when a draw first needs them, the pipelines of the last session
+// made ahead. what a task makes is taken over by the thread of the renderer when it's done (the frame after, as a rule); a task
+// itself touches only what it was given and what is guarded for it
+
+#pragma once
+
+#include <condition_variable>
+#include <cstddef>
+#include <deque>
+#include <functional>
+#include <mutex>
+#include <thread>
+#include <vector>
+
+namespace vk14
+{
+
+class background_tasks
+{
+  public:
+	background_tasks() = default;
+	background_tasks(background_tasks const &) = delete;
+	background_tasks &operator=(background_tasks const &) = delete;
+	~background_tasks();
+
+	void start(std::size_t Threads);
+	// the tasks under way are finished, the ones waiting dropped
+	void stop();
+	bool running() const
+	{
+		return false == m_threads.empty();
+	}
+	// Urgent: before the tasks waiting (what a draw waits for), otherwise after them (what's made ahead)
+	void add(std::function<void()> Task, bool Urgent = true);
+	// waits until there's nothing waiting and nothing under way
+	void wait_idle();
+	// the tasks waiting and under way
+	std::size_t pending() const;
+
+  private:
+	void work();
+
+	mutable std::mutex m_mutex;
+	std::condition_variable m_wake;
+	std::condition_variable m_idle;
+	std::deque<std::function<void()>> m_queue;
+	std::vector<std::thread> m_threads;
+	std::size_t m_busy{0};
+	bool m_stop{false};
+};
+
+} // namespace vk14