[FEATURE REQUEST] Support Geotiffs

**Is your feature request related to a problem? Please describe.**
I'm currently using OIIO to generate Big geotiffs (larger than 4GBytes).
I get difficulties to manage custom Tiif tags TIFFTAG_GEOTIEPOINTS, TIFFTAG_GEOPIXELSCALE) which don't seem to be a part of exif tags.
Moreover, previous code which allowed to write geotiff files used the method GTIFSetFromProj4 from Geotiff lib.
This method allows to update tiff tags according Proj4 projection.
Do you know if there is an elegant way to write such Geotiffs files using OIIO ?
I tried to firstly write the tiff using OIIO then update the header using GeoTiff lib but It doesn't seem to work.
Any idea ?

**Describe the solution you'd like**
A perfect solution would be to allow OIIO to manage as well tiff as geotiff format (using libtiff and geotiff)

No source files, tests, or entry points are identified in the issue. Start by locating OIIO's existing TIFF writer and reviewing how libtiff and GeoTIFF metadata could support TIFFTAG_GEOTIEPOINTS, TIFFTAG_GEOPIXELSCALE, projection data, and files larger than 4 GB. Done means OIIO can write GeoTIFF files with these tags and projection information.

Written by the indexing model from the issue text.