TODO
* add lens and cylindrical lens scene objects
* make surfaces's shadows filter the light of PhongSources through arbitrary colour filters instead of either completely
absorbing them or not absorbing them at all
* make shadow-throwing a property of a point on a surface rather than the surface property (so that tiled surfaces
can be partially shadow-throwing)
* complete implementation of transformation-optics-element surfaces
* add general quadratic surface [have now added paraboloid]; define an orthogonal coordinate system and formulate in terms of those coordinates
* implement surface property that makes rays nice and glowy interactively
* allow TIM to focus on objects at a *negative* distance (if none can be found at a positive distance)
* allow the transmission/reflection coefficient of *all* reflective/transmissive surfaces to be edited in the interactive version (not just of the transissive property)
* Fresnel reflection and transmission coefficients for refraction? Maybe this isn't the spirit of TIM, as it doesn't make any sense for METATOY surfaces, I guess
Known bugs
* sometimes EditableSceneObjectCollections just show up as black.
For example, the cubic BoxCloak, when converted to a collection of scene objects, turns black.
Strangely, though, if some subset of the windows is shown, e.g. only those in the outer cube, it works.
* no scroll bars appear when the content of a pane is too large to fit --- fix this!
* fix holographic teleporter
* eye remains at origin when camera position changes; make it follow the camera!
