Implemented point light shading. Almost working, still off in some the TransformStackTest.
Created Transformable common base class.
Implemented Light transformations.
Renamed DoLighting to Shade, and Shade to DoLighting.
Light calculations are working. Light calculations have been cleaned up. DirectionalLight now derives from PointLight to keep commanality in diffuse/specular contribution calculations.
Added some comments to raytracing color computation.