sum() values can wrap! Most systems only have 31 bits
available for positive integers. (Meaning, be careful
with images that have more than 8425104 pixels.)
average() and sumf() may also wrap, but on a line basis.
(Meaning, be careful with images that are wider
than 8425104 pixels.) These functions may have a precision
problem instead, during to limits in the 'double' C type and/or
'float' Pike type.