Fix compilation with Qt < 5.15

src/dialogs/markerdialog.cpp

@@ -115,7 +115,11 @@ void MarkerDialog::slotUpdateThumb()
 
 QImage MarkerDialog::markerImage() const
 {
+#if QT_VERSION < QT_VERSION_CHECK(5, 15, 0)
+    return clip_thumb->pixmap()->toImage();
+#else
     return clip_thumb->pixmap(Qt::ReturnByValue).toImage();
+#endif
 }
 
 CommentedTime MarkerDialog::newMarker()