More options to make-regtest-pngs.sh, use GNU parallel for faster conversion

More options to make-regtest-pngs.sh, use GNU parallel for faster conversion

Also rearranges the options slightly.  -p is now used for GNU parallel
instead of rendering PDFs via Cairo which is now done with -c.  The
whole kaboodle is now:

Make PNG files from regtests

Usage:  ./make-regtest-pngs.sh -j CPUs -o/-n

  where -j specifies the number of parallel processes to run
  (normally CPUs+1).  e.g.:

  ./make-regtest-pngs.sh -j9

 -p uses GNU parallel with the given job count in order to also
 parallelize the conversion of PDF files to bitmaps when using -g
 or -d.  No attempt is made to parallelize the bitmap comparisons
 since their memory requirements may be prohibitive.

 -o means build an old regtest set - the PNGs go in the old-regtest-results
 directory

 -n means build a new regtest set - the PNGs go in the new-regtest-results
 directory

 -c uses PDF and the poppler library via pdftocairo for generating
 bitmaps, simulating the output for Evince and other previewers
 using poppler.  pdftocairo may be contained in the poppler-utils
 package.

 -r can be used for specifying a rendering resolution.  This
 defaults to 101 for poppler and 300 for Ghostscript from PDF.

 -g uses Ghostscript for rendering a bitmap version from the PDF,
 simulating the output from printing PDF files on a GNU system, so
 use a resolution appropriate for print.  Antialiasing is not enabled.

 -d changes the Ghostscript device used for creating PNG files
 (usually png16m for direct PNG creation and pngmono for printer simulation)