Add Nuke txReader/txWriter plugin sources and build files

Add source and build code for Nuke plugins to read and write .tx files using OIIO. These were originally written at Luma against a much older version of OIIO, but have been updated to work with the current API.

Reader currently supports reading of a single mip level at the file's base resolution (limited to 4 channels at present).

Writer supports a subset of the `maketx` options, and is also currently limited to 4 channels due to what seems to be an issue with OIIO storing custom channel names (support for extra named channels is currently parked on another internal branch until it can be used).
Hi guys, can I ask if you're intending for the txReader to be used for actually 3D rendering in Nuke or simply for manipulation of textures to be written out again?  If rendering then using the standard scanline engine() method for texture access is horribly inefficient due to the default caching in Nuke which was not designed for that purpose.  There were nasty hacks added to the shading system to try and avoid the need for the 3D system to request the entire texture image which to my knowledge are still there.
For rendering purposes it's better to subclass the reader off Iop or Read and implement the Iop::sample() methods that the shading system uses.  Obviously that's a bit more work...
Food for thought.