portable: add return parameter to GetImageMetadataWithExtensions

The complaint was that the output array was used for two kinds of data, and the
input flag decided whether this extra data should be included. The flag is
removed, and instead the old method is changed to include the data always as
a separate parameter.

This breaks backward compatibility, but  the old method is effectively broken
and does not appear to be used yet, at least in open source code, by
searching on codesearch.debian.net and github.com.

Fixes #22404.

Co-authored-by: Luca Boccassi <bluca@debian.org>