Added new osg::Projection node, and osgUtil::NewCullVisitor which are work

in progress for the new support for controlling the projection matrix from
within the scene graph.

Also Added osg::State::getClippingVolume(), getProjectionMatrix() and
getModelViewMatrix() with allows drawables to access the current projection,
and model view matrices and the view frustum in local coords to the drawable.

src/osg/State.cpp

@@ -569,3 +569,11 @@ void State::haveAppliedAttribute(const StateAttribute::Type type)
         as.changed = true;
     }
 }
+
+ClippingVolume State::getClippingVolume() const
+{
+    ClippingVolume cv;
+    cv.setToUnitFrustum();
+    cv.transformProvidingInverse((*_modelView)*(*_projection));
+    return cv;
+}