GenericReader: bug fix, output is always premultiplied
ReadOIIO: premult: handle the case where alpha can be unassociated
GenericReader: fix premult
- NEVER read anything from the output image in an OFX plugin (@MrKepzie this should be in the OFX plugin development guide)
- make the mustPremult condition clear (cf. comments) and fix bugs in certain situations
GenericReader: Opaque images must not be (un)premult
GenericReader/Writer: fix numerous premult bugs + merge