For backwards-compatibility reasons, we can only add struct members to the
end. That way, existing callers still can use newer libraries, as the
structs are always allocated by the library (and therefore guaranteed to
have the correct size) and still rely on the same position of the parts
the callers know about.

Reported by Luca Falavigna.
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>