Another round of ImageBufAlgo API modernization

This time: compare, computePixelStats, isConstantColor, isConstantChannel, isMonochrome, fixNonFinite.

All now conform to the convention accepting an ROI (to provide a size to set up the destination, if uninitialized, or to limit the region or channels that the operation will be performed on) and a nthreads (controlling multithreading).  Not all actually multithread, some do, but even those that do not accept the parameter so that in the future they can be parallelized without having to modify the API again.

Also beefed up the comments for these functions to more precisely describe what they do.

This almost completely makes IBA functions have a fairly uniform interface style.  There are only one or two more stragglers left to do.