Better font searching.

Instead of searching for just one default font if none is specified,
have a list to search. Any one of them will prevent a total failure
of IBA::render_text().
TIF output support of Exif (only available for libtiff >= 4.0.2)

Earlier versions of libtiff (including the version that was current when
we added Exif reading to our tiff support) supported reading Exif, but
not writing Exif. Later versions of libtiff have added this capability.
More work on TIFF output of Exif.
Make TIFF error message storage thread-safe