add light source

There have to be light source feature to visualize 3d shapes. You cant get the sphere is the sphere if no light makes no shadow on the surface... and so on

The issue does not name files, tests, or an entry point. First inspect how Zdog renders shapes through its canvas and SVG paths, then clarify the lighting and shadow behavior required; done should mean 3D shapes can be visibly distinguished through a defined light-source feature.

Written by the indexing model from the issue text.