Fix mismatch between viewer FSAA methods and SoOffscreenRenderer

Open Inventor

The methods used for setting and getting the antialiasing fron the SoOffscreenRenderer and in Open Inventor viewers have different parameters making it very difficult to get the value from one of them and set it into the other one.

Now the SoOffscreenRenderer::setFullSceneAntialiasing is overloaded with the same parameter as the setFullSceneAntialiasing method in the viewers.