Smarter unassociated alpha handling.

1. Make IBA premult/unpremult (and oiiotool --premult/--unpremult) mark the IB's
metadata "oiio:UnassociatedAlpha" as dictated.

2. Make PNG output receiving a non-premultiplied buffer NOT preemptively unpremult.
(This makes it match Targa's behavior.)

This makes it somewhat harder to inadvertently double-unpremult, such as if you did:

    oiiotool in.exr --unpremult -o out.png