added comments.
major overhaul of the non-physical lights subsystem.

- the physical Sun's is (finally) fully functional.
- fixed directional light when instantiated in scaled assemblies.
- fixed multiple instancing of point, spot, directional and sun lights.
- renamed inputs of point, spot, directional and sun lights according to the actual physical quantities they represent.
added test scenes and updated test suite reference images.
Merge branch 'master' of github.com:appleseedhq/appleseed