Remove idiff restriction that compared images must be same size
ImageBufAlgo::compare_Yee overhaul and modernization.
* Take ROI and nthreads parameters like other modern IBA functions.
* Take CompareResults& parameter like IBA::compare() so it can report
  the pixel where the worst failure occurred.
* Use ImageBuf's, iterators, and IBA::convolve in many places where raw
  pointers and custom code to convolve and shuffle pixels around were
  previously used.