https://github.com/imageworks/OpenColorIO/pull/135
https://github.com/imageworks/OpenColorIO/issues/85

As OCIO is used to implement image displays, it was inconvenient that
the software processing path was unable to handle alpha channels.
This meant channel swizzling *almost* worked (it could handle viewing
all channels other than alpha).

This check-in adds an optional alpha interface to OCIO::ImageDesc, in
a manner that does not impact ABI compatibility. Also, the Nuke
ociodisplay node has been updated to now support channel swizzling
(including alpha, and alpha overlay mode).

-- Jeremy