TIFFWriteDirectory: Avoid overwriting following data if an IFD is enlarged.

Avoid overwriting other data if an IFD that has already been written is enlarged. If entries are to be changed in an IFD that has already been written to the file, the memory area can be overwritten as long as the IFD data volume has not increased. Otherwise, the IFD must be written to a different location in the file. Up to now, LibTIFF has simply overwritten the IFD and other data areas in the file.

With this MR, the size of the IFD is tracked and checked if the IFD can be safely overwritten or has to be re-written to another location.