MuseScore visual tests

The shell script "gen" creates a subdirectory with the
html file "vtest.html" for visual compare including all needed
image files.

Add a new test:
      - clone a mscz test file into xxx.mscz
      - create the test by editing the xxx.mscz file
      - create reference png xxx-ref.png:
            mscore xxx.mscz -o -r 130 xxx-ref.png