FR: Generate better diff images

[tingerrr]

When generating a diff image, the following things could be improved:

• generate diff images on a page size mismatch (Diff images are not being generated for page size differences #8)
• highlight changes, bounding boxes of small change sets can be created very easily using a border finding algorithm (see for example Suzuki and Abe ¹)
• highlight which kind of change it is
  • red: deletion from reference to output
  • green: addition from reference to output
• overlay these boxes on the output or reference image to make it easier to visually compare

Footnotes

1. https://www.nevis.columbia.edu/~vgenty/public/suzuki_et_al.pdf ↩