Centroids challenge:

Modify the code reg-grow.c (linked at website) to calculate and print
out the centroid for every region it finds.
Run it on targets.ppm and verify it finds the centroids of the squares
on the calibration target.

extra challenge:

Change the region size limit(s) after the region grow to try to weed
out all returned regions except for the 8 squares on the calibration target.