From Pjotr Svetachov, "I think I spotted some unneeded ref_ptrs in the cullvisitor. The call
pushModelViewMatrix or pushProjectionMatrix will already keep the
reference when adding it to the MatrixStack. In CullVisitor::apply
methods for the billboard and the camera you already take a pointer
instead of a ref_ptr."


git-svn-id: http://svn.openscenegraph.org/osg/OpenSceneGraph/trunk@14375 16af8721-9629-0410-8352-f15c8da7e697