- pix-min-or-maxprocedure
- in - PPIXD PPIX1 - #f or PPIX1 (see NOTE above)
- PPIXD (destination ppix) is optional, if used, must be used with PPIX1. If #f, a new ppix is alloc and returned (typical case). Use PPIX1/PPIX1 for in-place modification of PPIX1.
- in - MIN-MAX - select via CHOOSE-MIN or CHOOSE-MAX
- returns: PPIXD (new alloc if needed) with min/max pixel values, #f if error.
- in - PPIXD PPIX1 - #f or PPIX1 (see NOTE above)