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application to add a new attribute, and tell the old applications how to
interpret this by adding the old attribute until all apps are updated.

> Works for us! If anyone has objections, please raise them now.

Summary:
- use visiblilty=visible|hidden
- missing visibility ==> visible
- unknown value      ==> visible

If nobody objects, let's just do it that way. We can still make a forward
compatible extension with that, if anyone else cares.

> Would it be acceptable to ignore the alpha-locked attribute in an
> application?

I think so, because it doesn't affect how the image looks.  Would be nice if
the saved ORA would preserve the value, I guess.