oiiotool --pdiff is perceptual diff equivalent to idiff -p

Not especially purposely, oiiotool --diff did not fully replicate the original functionality of 'idiff'. In particular, there was no prevision for a perceptual diff. Added now as --pdiff.

Also rigged it so that either --diff or --pdiff, when images fail to match closely enough, will set the oiiotool run status code to be nonzero.