plug-ins: create file-pdf-save2 with the additional parameter.
And revert file-pdf-save to its previous signature before commit d03de527. For compatibility reason, we can't change the signature of a procedure available in previous stable releases. Also revert file-pdf-save-multi for the same reason. I don't create a file-pdf-save-multi2 though even though layers-as-pages could still be meaningful there (creating a multi-page PDF from several multi-layers images) but that's likely a special case which can be dealt with later. Obviously the interactive export will use file-pdf-save2.
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