Introduce realtime-mode comparisions.

This involves:
- Adding the "--preset" as a tunable parameter to x264
- Modifying the generation scripts to generate "rt" comparisions
- Modifying the generation scripts to ignore results that aren't
  good enough according to the scorer (score is negative)
- Adding tools to run a specific parameter set across all video files
- Fixing various bugs and misfeatures encountered on the way
This interface is hard to understand. mpeg_settings.files is a mapping between "class names", whatever that is, and filenames? Also, since you have a key and are looking things up in various maps, it seems you would be well served by objects here. I'm not sure what such an object would be called though. EncodedMpegFile, since it has a file and bitrates? You know better here, I'm not sure what the bitrates mean in this context.

Anyway, Then you could write this as
for encoded_mpeg_file in mpeg_files.GetAllEncodedMpegFiles():
  videofile = encoded_mpeg_file.AsVideoFile(encoder)
  # Or videofile = encoder.Videofile('video/mpeg_video/' + encoded_mpeg_file.filename)
  for rate in encoded_mpeg_file.rates():
    ...