- pix-gammaprocedure
- in - PPIXD - #f or PPIX (see NOTE above)
- in - PPIX
- in - GAMMA - 1.0, no change, <1.0 darker, >1.0 lighter
- in - M* in - MIN usually >=0, but may be < 0–black will be gray.
- in - MAX - MAX usually <= 255, but may be > 255, then whites are gray…
- returns: modified ppix or #f if error.