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).
That's something the previous incarnation of the plugin tried to do, written against an older OIIO API. When I updated the plugin, I removed that behavior as a test and found that the one-shot approach was orders of magnitude faster and more memory-efficient than the old plugin, so I decided to leave that as a low-priority item (along with implementing the planar Reader interface for Nuke 8).

I believe the biggest impediment to more efficient region reading is that, to my knowledge, a Reader can't get access to the full box that was requested from the Read that owns it, so while you can get an X-R bound from the scanline you're being asked to fill, you don't really knob how many lines need to be read. Since file I/O is always going to be the biggest bottleneck, I figured trying to do lots of piecemeal reads would prevent the OIIO underpinnings from operating at peak efficiency.

If people feel strongly enough about it at this point, however, I can certainly revisit it.