Converted AnimationPath so it is now subclassed from osg::Transform::ComputeTransfromCallback()
so that it can be directly attached to the scene to move objects about.
Changed the osg::SateSet::setGlobalDefaults() so that is sets GL_DEPTH_TEST
to ON.
Added guards and creation of default helper classes in osgGLUT::Viewer and
osgUtil::SceneView so that sensible defaults are used when no settings
are used of osg::State/StateSet,osgUtil::RenderStage/RenderGraph/CullVisitor.
