Invoking lilypond using the read-file-list option and job-count of 9, I get
output image files of varying sizes. This is messing up my pixel comparator
regtest checker. I've tracked it down to the use of
#(set-default-paper-size "A6") in one of the regtests. Once this has been
used, it changes the default paper-size for all output created with that
instance of lilypond. I've read the NR and I know it's supposed to affect
the whole of the scope it applies to, but not surely all the subsequent
files in a multi-run job?