[qupgtxbw] Ray tracing with conic sections

The Zen Photon Garden has a single point light source and partially transparent flat mirrors, everything in 2D.

Other things to add: different refractive indices (to make lenses), shapes curved like conic sections, parallel rays as a light source (though parallel rays could easily be constructed from a point source and parabola).  We need a more sophisticated UI.

Since the shapes are simple, can the "garden" be computed directly instead of ray tracing?  Multiple reflections probably make things difficult.

Much more difficult: splitting and combining colors of light, diffraction.