- pix-get-pixel-avgprocedure
- in - PPIX
- in - PPIXM - (optional) 1bit mask of area to measure, may be #f
- in - X Y - UL coords of mask relative to PPIX - ignored if PPIXM is #f
- in - FACTOR - subsampling factor 1, 2, 4
- out - PVAL: ptr to unsigned-int - avg pixel val
- returns: 0 if “OK”, 1 on error.