Added new osg::GraphicsContext base class

src/osg/State.cpp

@@ -21,6 +21,7 @@ using namespace osg;
 
 State::State()
 {
+    _graphicsContext = 0;
     _contextID = 0;
     _identity = new osg::RefMatrix(); // default RefMatrix constructs to identity.
     _initialViewMatrix = _identity;