There are two examples in the code. The first one shows how to implement a GTK+ viewer for OpenSceneGraph using the C++ version of GtkGLExt. The second builds on this to show a modified osgterrain demo with a WebKit-rendered page instead of the terrain texture. To compile this code you’ll need to patch WebKit to support off-screen cairo rendering, you can get the patch at the OpendedHand bugzilla.
As most of the demo code I’ve been publishing lately, it’s more of a proof-of-concept than working code, so handle with care.