Premultiplied alpha not rendered correctly

[jonsneyers]

In the case of premultiplied (associated) alpha, djxl and other applications ignore that image header field, and treat everything as if it is the usual non-premultiplied (unassociated) alpha like in PNG.

It would make sense to do the same thing here as with the Orientation: by default, the decoder should return pixels in a default way (orientation corrected, unassociated alpha), so applications that want to interpret the alpha_associated info themselves and get the raw data would have to explicitly ask the decoder to not do the normalization.