This currently distorts the output size because Tesseract assumes it
knows the DPI better than we do.
Does not work for Ghostscript, because it emerges that Ghostscript
honors /UserUnit for rasterizing but not in pdfwrite (resolve/wontfix).
https://bugs.ghostscript.com/show_bug.cgi?id=690781
Ghostscript’s output would need to be patched in a PDF/A safe way for
this to work. Temporary route may be to block Ghostscript if
/UserUnit.