avoid looping badly on IFD loops in TIFF images, assuming a recent enough libtiff

avoid looping badly on IFD loops in TIFF images, assuming a recent enough libtiff

Switch to TIFFReadDirectory() instead of TIFFSetDirectory(), since
TIFFReadDirectory() does IFD loop detection.

Unfortunately it doesn't return an indication that there *is* a loop,
it just indicates the end of the chain, so we can't warn the caller
about the problem.