Suppressed Reading of Tiff tags with ID = 0 (like GPSVERSIONID) corrected.

Issue: Custom Tiff tags with ID=0 are not read.

Correction: IGNORE placeholder in tif_dirread.c is now replaced by a field dir_ignore in the TIFFDirEntry structure.

Explanation:
Currently, in tif_dirread.c a special IGNORE value for the tif tags is defined in order to flag status preventing already processed tags from further processing. This irrational behaviour prevents reading of custom tags with id code 0 - like tag GPSVERSIONID from EXIF 2.31 definition.

An additional field 'tdir_ignore' is now added to the TIFFDirEntry structure and code is changed to allow tags with id code 0 to be read correctly.

This change was already proposed as pending improvement in tif_dirread.c around line 32. (Joris Van Damme 27.03.2007)

Reference is also made to:
- Discussion in !39 (closed)
- http://bugzilla.maptools.org/show_bug.cgi?id=2540