Move ray.h, ray-io.h, and ray-io.cc into geometry subdir
Move isec-cache.h into space subdir
Move intersect.h and intersect.cc into render subdir
Remove intersect subdir

It is no longer used.
Move camera-related code into a new subdirectory "camera"
Properly quote string-contents in update-build-info.sh
Move scene-cmdline.lua into top directory

Despite the name, it's not really related to other stuff in the scene
subdirectory.
Move scene.{h,cc,lua,swg} code into render subdir

The Scene class is really a render-time only interface (as opposed to
the general scene description), and putting it with the rest of the
rendering code makes that a bit more clear.
Remove scene subdir

It is no longer used.
Use struct/class consistently