`output-distance.py` improvements

• output-distance.py: Better HTML source code readability

  • Empty lines only between regression tests.
  • Add div class 'invisible' for better separation between contents and style.

• output-distance.py: Redesign HTML appearance

  • Instead of doing a blink comparison between the old-and-overlayed-diff image and the old image on the right side, do a blink comparison of the old and new image on the left side.
  • The button now displays 'before' by default and 'after' while being activated.
  • Synchronize vertical distances between the 'before' and 'after' columns.
  • No longer add links to images.

• output-distance.py: Improve HTML layout

  • Give unchanged images a border.
  • Improve vertical spacing between images.