Current colour averaging scheme is outdated

[mgbellemare]

The current colour averaging scheme is 4 years old and maps two NSTC frames to a NTSC frame. This implies that the resulting colour space is smaller than it should be. The actual quantization (going from NTSC -> RGB -> NTSC) is also crude.

Without deprecating the existing scheme (which serves the purpose, e.g., of providing frames drawn from a small colour space), it would nice to bring it up to date, perhaps avoiding the re-conversion to NTSC and instead producing a new "average-NTSC" colour space. Alternatives include providing proper RGB averaging routines, but these should probably be done in agent space.