Have made osg::Transform more extensible via additions of new getLocalToWorldMatrix()

and getWorldToLocalMatrix(), computeLocalToWorld() and computeWorldToLocal()
methods.

Have updated the CullVisitor, IntersectVisitor and Optimizer to use the
new osg::Transform::getLocalToWorldMatrix() which has the same functionality
as the old getMatrix() but is now supports subclasses of osg::Transform
transparently.

Have added osg::PositionAttitudeTransform as subclass of osg::Transform
which manages the transform as position and attitude via a Vec3 and Quat
respectively.

src/osgUtil/CullVisitor.cpp

@@ -1054,7 +1054,7 @@ void CullVisitor::apply(Transform& node)
     StateSet* node_state = node.getStateSet();
     if (node_state) pushStateSet(node_state);
 
-    pushCullViewState(&node.getMatrix());
+    pushCullViewState(&node.getLocalToWorldMatrix());
 
     traverse(node);