videorec: Fix flushing encoders
When passing NULL frames, encoders empty their queues, but we have
to stop polling from them when they don't any more data.
#include <QApplication>
#include "mainform.h"
int main(int argc, char *argv[])
{
QApplication app(argc, argv);
MyWindow *mainWin = new MyWindow;
mainWin->show();
return app.exec();
}