Whether an overlay could be established for the current window given the current initial display mode . If false, glutEstablishOverlay will fail with a fatal error if called. If the current window has an overlay established. Calling glutPostRedisplay will not set this true. Calling glutPostRedisplay or glutPostOverlayRedisplay will not set this true.
Negative one is returned if no overlay is in use.
Description glutLayerGet retrieves GLUT layer information for the current window represented by integers. The info parameter determines what type of layer information to return.