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
Fair enough; please make a follow-up CL later.