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
The MPEG file set needs to become an instance of a video file collection.
For each clip, there are a set of bitrates that make sense, and there are many pieces of the code where we iterate over all the sensible filename/bitrate combinations; this iteration should be embedded in the collection object.

However, this is a change that should not be part of this CL; this CL just contains 2 more examples of the pattern that is already prevalent.