Cleanup existing native color space conversions.

Cleanup existing native color space conversions.

This change clarifies that the existing conversions are according to the
ITU 601 standard and scaled to and from full gamut RGB. Also, adjust 2
coefficients according to Charles Poynton's matrices. This does not yet
attempt to make any substantial improvements.

Finally, it replaces the verbose logic and redundancy in the image
conversion routine with a concise function dispatch table.