Fixes a z-prefix class name that I forgot about.

This is quite embarrassing. It should be a major revision change, since
anybody who's been relying on CSound2D now has a broken implementation,
but honestly, who's using it? Nobody. So I'm pushing this as a critical
quick-fix patch revision. Ain't nobody gonna care.
Fixes scene lighting when no geometry is present.

TODO: Add a parameter to specify scene clearing color.
Adds a couple of TODO items.