Improve support for TIFF/EP (DNG)
There are a number of tags that are identical for Exif and TIFF/EP, the only difference that TIFF/EP stores them in the root IFD.
Currently, libtiff complains about adding these Exif/TIFF-EP tags to any IFD other than an Exif IFD, which hinders easy creation of TIFF/EP compliant files like DNGs which do not have an Exif IFD.
Also, some tags specific to TIFF/EP (or ones that differ to Exif) would need to be added.
https://en.wikipedia.org/wiki/TIFF/EP
http://www.barrypearson.co.uk/top2009/downloads/TAG2000-22_DIS12234-2.pdf
Alternatively, just lift (or provide the mechanism to work around) the Exif IFD restriction without adding anything else.