Dive media: Extract thumbnails from videos with ffmpeg

Extract thumbnails using ffmpeg.
Behavior is controlled by three new preferences fields:
 - extract_video_thumbnails (bool): if true, thumbnails are calculated.
 - extract_video_thumbnail_position (int 0..100): position in video
   where thumbnail is fetched.
 - ffmpeg_executable (string): path of ffmpeg executable.

If ffmpeg refuses to start, extract_video_thumbnails is set to false
to avoid unnecessary churn.

Video thumbnails are marked by an overlay.

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

core/videoframeextractor.h

@@ -0,0 +1,37 @@
+// SPDX-License-Identifier: GPL-2.0
+#ifndef VIDEOFRAMEEXTRACTOR_H
+#define VIDEOFRAMEEXTRACTOR_H
+
+#include "core/units.h"
+
+#include <QMutex>
+#include <QFuture>
+#include <QThreadPool>
+#include <QQueue>
+#include <QString>
+#include <QPair>
+
+class VideoFrameExtractor : public QObject {
+	Q_OBJECT
+public:
+	VideoFrameExtractor();
+	static VideoFrameExtractor *instance();
+signals:
+	void extracted(QString filename, QImage, duration_t duration, duration_t offset);
+	// There are two failure modes:
+	//	failed() -> we failed to start ffmpeg. Write a thumbnail signalling "maybe try again".
+	//	invalid() -> we started ffmpeg, but that couldn't extract an image. Signal "this file is broken".
+	void failed(QString filename, duration_t duration);
+	void invalid(QString filename, duration_t duration);
+public slots:
+	void extract(QString originalFilename, QString filename, duration_t duration);
+	void clearWorkQueue();
+private:
+	void processItem(QString originalFilename, QString filename, duration_t duration);
+	void fail(const QString &originalFilename, duration_t duration, bool isInvalid);
+	mutable QMutex lock;
+	QThreadPool pool;
+	QMap<QString, QFuture<void>> workingOn;
+};
+
+#endif