Added computation of the bounding volume of osg::OccluderNodes.

Added support for osg::BoundingSphere::expandBy*(osg::BoundingBox) and have
added osg::BoundingSphere/Box::valid() which deprecates isValid(), this
is to be more consistent with other classes in the OSG.

src/osg/OccluderNode.cpp

@@ -11,3 +11,27 @@ OccluderNode::OccluderNode(const OccluderNode& node,const CopyOp& copyop):
     _occluder(dynamic_cast<ConvexPlanerOccluder*>(copyop(node._occluder.get())))
 {
 }
+
+const bool OccluderNode::computeBound() const
+{
+    bool result = Group::computeBound();
+    
+    if (getOccluder())
+    {
+        BoundingBox bb;
+        const ConvexPlanerPolygon::VertexList& vertexList = getOccluder()->getOccluder().getVertexList();
+        for(ConvexPlanerPolygon::VertexList::const_iterator itr=vertexList.begin();
+            itr!=vertexList.end();
+            ++itr)
+        {
+            bb.expandBy(*itr);
+        }
+        if (bb.valid())
+        {
+            _bsphere.expandBy(bb);
+            _bsphere_computed=true;            
+            result = true;
+        }
+    }
+    return result;
+}