small fix in viewer

Former-commit-id: e5859cc80077270494a954160b2f1b1e75b497fd

include/igl/viewer/Viewer.cpp

@@ -867,6 +867,7 @@ namespace viewer
     launch_init(resizable,fullscreen);
     launch_rendering(true);
     launch_shut();
+    return 1;
   }
 } // end namespace
 }