void draw() {
     glClear(GL_COLOR_BUFFER_BIT);
     for_each_eye([&](StereoEye eye) {
         renderEye(eye);
     });
     GL_CHECK_ERROR;
 }
 void draw() {
   glClear(GL_COLOR_BUFFER_BIT);
   for (int eyeIndex = 0; eyeIndex < 2; ++eyeIndex) {
     renderEye(eyeIndex);
   }
   GL_CHECK_ERROR;
 }