Add scene files

[jalberse]

We'll want something like this: https://www.cs.utexas.edu/users/fussell/courses/cs384g-fall2011/projects/raytracing/fileformat.html

This allows us to define a scene in a file, which we can then pass to the renderer.

We should replace the main.rs scenes by just providing sample scene files.

[jalberse]

I think the ideal would be to implement #53 to get USD support. This ticket would be to get a minimal thing off the ground for development purposes, so I don't have to deal with USD's complexities while still being able to define my own scenes.

I could for example probably write a Blender python script to export simple scenes to my file format.

.shm might be a good file extension if I want to be cute about it.