This adds a new `tif_lastdiroff` field to the TIFF data structure
and uses it to store the offset of the last written directory.

Appending a new directory required traversing the whole file
to find the last directory. By keeping track of its offset in this
new field, the search is no longer necessary.

Since this offset is only stored in-memory, the first directory
append after opening a file will have to transverse the whole
directory list. Subsequent calls will have access to the last
offset, avoiding the transversal.