Moved the code referencing osg::BoundingBox::isValid() and

osg::BoundingSphere::isValid() across to use the valid() methods, the later
being more consitent with other classes such as osg::ref_ptr<>.

include/osg/CullStack

@@ -102,7 +102,7 @@ class SG_EXPORT CullStack
 
         inline bool isCulled(const BoundingBox& bb)
         {
-            return bb.isValid() && _modelviewCullingStack.back()->isCulled(bb);
+            return bb.valid() && _modelviewCullingStack.back()->isCulled(bb);
         }
         
         inline bool isCulled(const BoundingSphere& bs)