Use Data::DocumentMedia to store good thumbnails.

Telegram/SourceFiles/data/data_document_media.h

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+/*
+This file is part of Telegram Desktop,
+the official desktop application for the Telegram messaging service.
+
+For license and copyright information please follow this link:
+https://github.com/telegramdesktop/tdesktop/blob/master/LEGAL
+*/
+#pragma once
+
+#include "base/flags.h"
+
+namespace Data {
+
+class DocumentMedia final {
+public:
+	explicit DocumentMedia(not_null<DocumentData*> owner);
+	~DocumentMedia();
+
+	void goodThumbnailWanted();
+	[[nodiscard]] Image *goodThumbnail() const;
+	void setGoodThumbnail(QImage thumbnail);
+
+	// For DocumentData.
+	void validateGoodThumbnail();
+	static void CheckGoodThumbnail(not_null<DocumentData*> document);
+
+private:
+	enum class Flag : uchar {
+		GoodThumbnailWanted = 0x01,
+	};
+	inline constexpr bool is_flag_type(Flag) { return true; };
+	using Flags = base::flags<Flag>;
+
+	static void ReadOrGenerateThumbnail(not_null<DocumentData*> document);
+	static void GenerateGoodThumbnail(not_null<DocumentData*> document);
+
+	const not_null<DocumentData*> _owner;
+	std::unique_ptr<Image> _goodThumbnail;
+	Flags _flags;
+
+};
+
+} // namespace Data