UI: Try closing remux before initiating shutdown

Makes OBS try to close the remux dialog before Clearing Scene data since
in case a remux might still be going on and the user does want to abort
closing OBS.

UI/window-basic-main.cpp

@@ -4676,6 +4676,12 @@ void OBSBasic::closeEvent(QCloseEvent *event)
 		}
 	}
 
+	if (remux && !remux->close()) {
+		event->ignore();
+		restart = false;
+		return;
+	}
+
 	QWidget::closeEvent(event);
 	if (!event->isAccepted())
 		return;