osg::PositionAttitudeTransform::AnimationPathCallback which us an app callback
which uses an AnimationPath to specify the new positions of the transform.
Added AnimationPath code to osglight to animate the positional light.
Added CullVisitor::setCullingMode() code into SceneView so that SceneView's
CullingMode is now properly passed onto CullVisitor. Note, this work
had been done before, but must has been lost in a merge. Umm...
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.
