Convert PDF to PNG and compare PNG files after l3build
Description
The build system l3build normally writes the contents of some
boxes from .lvt files into corresponding .tlg files. Sometimes
a dependent package adds e.g. the command \kern0pt, so that
test files fail, even if the PDF files look the same as before
and are still correct. The ppmcheckpdf tool offers an
alternative option for regression testing: instead of printing
the contents of boxes in .lvt files, PDF files are converted to
PNG files and you can compare the PNG files after l3build has
finished its work.
