This will significantly reduce CPU usage if the video source larger than
the selected area and make it easier to only check parts of a video
source output image.
This is a workaround to handle random segfaults triggered when using ...
QMetaObject::invokeMethod(this, "RedrawImage", -Qt::QueuedConnection,
Q_ARG(QImage, image));
... from within CheckForMatchLoop().
Even using BlockingQueuedConnection causes deadlocks.
This should avoid issues of the whole plugin not loading if OpenCV is
not available.
This approach should also work for possible future external
dependencies.
