Add option to being able to perform basic transformation on images (rotate/mirror)

[martonmiklos]

While working on #418 I got an idea: I used to create images in left and right variants for different connector orientations:

It would be great if the wireviz would provide option least for horizontal mirroring of the images. If we there at the implementation rotating vertical mirroring too.

Comments, concerns, ideas welcome in this topic!

[kvid]

I do see the usefulness in such simple image processing (mirror, rotate, and maybe crop), and it isn't too hard to do each of them, but we also need to create and write the result to a temporary image file with a name that doesn't conflict with other names reachable through the image_paths - and optionally delete it again in a final clean-up step when no longer needed. A similar need is also described in #322 (comment) - a common code should be created.

If more than one processing step should be allowed for the same image, then their order must be specified, and I guess a list of processing steps is the most flexible in that case, e.g. something like this:

    image:
      ...
      process:  # or "processing"?
        - crop: 0-100x0-200
        - flip: left-right
        - rotate: 180

Such a syntax could also allow a single processing step as an alternative to a list, similar to what #292 suggests for the image attribute.

Do you know any other additional image processing steps that might be requested in the future?