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 glutPos- tRedisplay will not set this true. Calling glutPos- tRedisplay or glutPostOverlayRedisplay will not set this true.
Negative one is returned if no overlay is in use.
The info parameter determines what type of layer information to return.