Smarter OpenEXR channel ordering for files with ZBack, RA, GA, BA

OpenEXR stores channels alphabetically (!). OIIO's exr reader rearranges the channel order upon reading so that it tends to resemble the "RGBAZ" order typically found in all other formats (and typically expected from image-reading software). This patch makes it smarter about ZBack, RA, GA, BA channel that are typically found in deep exr files.