Cleanup: unserialize loading of pictures

The images to load are kept in a set to avoid multiple loading of
the same picture. The set was protected with a mutex.

Problem: the mutex was not released before loading the picture,
effectively serializing the loading of pictures.

Therefore unlock the mutex once the set is updated.

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

core/imagedownloader.cpp

@@ -89,6 +89,8 @@ static void loadPicture(struct picture *picture, bool fromHash)
 	if (queuedPictures.contains(QString(picture->filename)))
 		return;
 	queuedPictures.insert(QString(picture->filename));
+	locker.unlock();
+
 	ImageDownloader download(picture);
 	download.load(fromHash);
 }