workspace.save_image_file: set dpi on image.save, #343

[kba]

[bertsky]

Another idea I had: couldn't we at least try to force PIL.Image objects we ourselves produce (Workspace.image_from_page and image_from_segment) to retain or piggy-back the DPI info? Or even store that info in the workspace instance (e.g. by attaching the OcrdExif to the OcrdFile of the PAGE-XML, which itself could be cached in the OcrdMets; this would require rewriting Processor.input_files, but allow reusing these iterations)?