generic-tiff: Fix directory skew when a directory is ignored
generic-tiff: Fix directory skew when a directory is ignored

When mapping slide layers to TIFF directories, the generic TIFF driver
was not accounting for directories it had chosen to ignore.  For TIFFs
containing unrelated directories between valid slide layers, this could
cause OpenSlide to return incorrect image data or to refuse to open the
slide.

openslide/openslide,openslide/openslide,openslide/openslide,openslide/openslide