Performance improvement by omitting unnecessary images from the Zip archive

The XML sources contain links to images supplied in the same folder as the XML sources. The improved pipeline has to make sure that all references are valid, i.e. every link goes to an existing image. The batch simply copied all images from the source folder to the result folders. The XProc 3.0 pipeline should be able to copy just the referenced images, but only the images referenced in the XML results, not in the XML sources. That is because there are cases when the pipeline omits the XML result because of special XPath conditions in the source or because the generated XML result is not valid against its specific version of T0 DTD. Although these additional unnecessary images are subsequently ignored by the REST services, it would be better to just omit them. This would result in smaller Zip archives and therefore faster transmission to the REST service and faster processing.