always an alpha channel

[Cyrille Berger]

> And the function of the eraser becomes different... On the other hand, with
> the current code, it's hard to add an alpha channel to an image that
> doesn't have one. Maybe always have one that's dormant.

We might separate the alpha channel from the other data, and put it in an 
other layer that can have either one channel or the exact number of channels.

-- 
--- Cyrille Berger ---