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/osg/PositionAttitudeTransform.cpp

@@ -0,0 +1,43 @@
+#include <osg/PositionAttitudeTransform>
+
+using namespace osg;
+
+PositionAttitudeTransform::PositionAttitudeTransform()
+{
+}
+
+void PositionAttitudeTransform::computeLocalToWorld() const
+{
+    if (_localToWorldDirty) 
+    {
+        if (_mode==MODEL)
+        {
+            _localToWorld->makeRotate(_attitude);
+            _localToWorld->setTrans(_position);
+        }
+        else
+        {
+            _localToWorld->makeTranslate(-_position);
+            _localToWorld->postMult(osg::Matrix::rotate(_attitude.inverse()));
+        }
+        _localToWorldDirty = false;
+    }
+}
+
+void PositionAttitudeTransform::computeWorldToLocal() const
+{
+    if (_worldToLocalDirty) 
+    {
+        if (_mode==MODEL)
+        {
+            _worldToLocal->makeTranslate(-_position);
+            _worldToLocal->postMult(osg::Matrix::rotate(_attitude.inverse()));
+        }
+        else
+        {
+            _worldToLocal->makeRotate(_attitude);
+            _worldToLocal->setTrans(_position);
+        }
+        _worldToLocalDirty = false;
+    }
+}