GitLab

 - convert gegl-tester output to TAP format
 - add optional test using gegl-tester

- set reference hash to unstable for operations that do not produce
  reproducible hashes.

consistent with the default composition.

- new reference compositions to better illustrate operation behaviour

reference chain.

- hashes taken from CI build

 - gegl.css is included in asciidoc generated files and does not need to
   need to be copied with htmlfiles.
 - add formatting elements for literalblock and listingblock classes.