ExportProjectDialog: fixed crash when exporting whole project as one track

We have to use the new semantics of the ProjectRenderer management when
exporting the whole project as one track as well. Otherwise the program
crashed as it tried to utilize the ProjectRenderer instance twice.

src/gui/export_project_dialog.cpp

@@ -308,7 +308,8 @@ void exportProjectDialog::startBtnClicked()
 	}
 	else
 	{
-		render(prepRender());
+		prepRender();
+		popRender();
 	}
 }