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/IntersectVisitor.cpp

@@ -555,7 +555,7 @@ void IntersectVisitor::apply(Transform& node)
 {
     if (!enterNode(node)) return;
 
-    pushMatrix(node.getMatrix());
+    pushMatrix(node.getLocalToWorldMatrix());
 
     traverse(node);