CameraGroup class for managing multiple cameras.

CameraGroup class for managing multiple cameras.

CameraGroup supports cameras opened in different windows or in the
same window, with flexible view and perspective specification.

Clean up mouse click handling via osgViewer. Don't let any camera
"have focus;" this forces events to be reported in window coordinates
and simplifies life. Don't use the osgViewer::View::requestWarpPointer
method; instead use our own code which only deals with window
coordinates.

Make glut and sdl versions work with CameraGroup too.