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).
I'm fairly certain `open()` is only called when it's actually time to do work. Readers don't deal directly with the `request()` phase (since it's being called on the parent Read), and the image info is populated in the Reader constructor rather than a separate `validate()` method (they get created and destroyed a lot). The only other things you can really implement on a Reader to do work are `open()` and `engine()`, and the header comments seem to indicate that the former is a safe place for this kind of thing:

    This is called before the first call to engine(). It should do any
    expensive setup that is not needed for setting the info() [...]

That last bit also seems to imply that `validate()` has already been called on the parent Read by the time `open()` is called. I'll poke around with this a bit more tomorrow and confirm that things work the way I suspect they do just to be safe.