If watermark applied to _original, then "Rebuild derivatives" watermarks all derivatives

Issue Summary

Hello,
AS a matter of fact, if we apply the watermark to _original image on creation, then rebuilding derivatives will make derivative images base on this watermarked _original, and all derivatives will get a watermark.
I guess this is "by design", but wouldn't there be a solution, by rewriting part of the module?

Thanks,
David