ptc40.ps: ghostscript reports: undefined -21
See https://github.com/mate-desktop/atril/issues/506 for an example file http://pages.cs.wisc.edu/~yeoh/nt/ptc40.ps.bz2 that renders fine without warning or error on GPL Ghostscript 9.50 (2019-10-15) (current Ubuntu 20.04), but fails to render with ghostscript reports: undefined -21
in libspectre1-0.2.8-2 as called by atril-1.24.0-1 (current Ubuntu 20.04).