Expose pose of scenery models to Nasal load/unload scripts.

Expose pose of scenery models to Nasal load/unload scripts.

This allows Nasal code attached to scenery models access their
world position and orientation by calling _model.getPose().

Also ensure models are attached to the scenegraph before Nasal
load scripts are called. This ensures that the world position
and orientation are available upon executing the load script.