v0.3

    + Implement multi-threaded rendering (-j/--threads).

    + Add photo-tracing renderer (-Sphoton).  The quality isn't so
      great, but it's a start.

    + Rewrite the environment-map light (EnvmapLight) to sample the
      environment-map exactly (as well as having cleaner and smaller
      code).  This makes environment-map lighting less noisy (as the
      PDF exactly matches the illumination).

    + Add "sampleimg" utility, which outputs a "dithered" version
      of an input image.


v0.2

    + Rewrite much of the rendering infrastructure to allow more
      flexibility in implementing different rendering methods.
      The -S/--surface-integ option allows choosing different renderers.

    + Add a real path-tracing renderer (-Spath).

    + Add support for better random-number generation, using std::random
      or boost::random if possible.


v0.1

    Original version
