message: highui: qt - enable hidpi by default
  can be overriden by environment:
  QT_AUTO_SCREEN_SCALE_FACTOR=0
diff: @@ -529,6 +529,9 @@ static int icvInitSystem(int* c, char** v)
  //"For any GUI application using Qt, there is precisely one QApplication object"
  if (!QApplication::instance())
  {
  +#if QT_VERSION >= QT_VERSION_CHECK(5, 6, 0)
  + QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling, true);
  +#endif
  new QApplication(*c, v);
  setlocale(LC_NUMERIC,"C");
  