Support for large number of IFDs

In life-sciences it is often to have files with hundreds of thousands of directories. Although it is obviously inefficient and should not be done with TIFF, it does exist. I maintain libbioimage that makes extensive use of libtiff to support many TIFF derived life-science formats and for years I've had my own branch always patched to support larger numbers of directories. I know it's an API change but don't think it makes a huge impact. Hope you can accept this pull request so I don't have to maintain my own branches anymore.