removal of node which could have been removed.
Added so extra convinience methods to ClippingVolume and utilized them in the
CullVisitor to push and pop only culling planes that are active.
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.
