ReadOIIO: don't let OIIO associate alpha, since it causes too much precision loss on 8bit TIFF unpremultiplied images
- see also https://github.com/OpenImageIO/oiio/issues/960
GenericReader: better doc for the Image Premult parameter
ReadOIIO: add heuristic to guess the alpha channel

If there are exactly 4 channels in the considered layer/view and 3 of
them are R,G,B, then consider that the fourth one is Alpha.