Current colour averaging scheme is outdated

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.

No files, tests, or entry points are named. Start by locating the existing colour averaging and NTSC-to-RGB-to-NTSC conversion implementation, then determine the intended average-NTSC or RGB behavior and how it should be validated without removing the existing scheme.

Written by the indexing model from the issue text.