Suggested imagehash rework option

[wjbenfold]

🚀 Pull Request

Description

Proposal of how to rework graphical testing, based on discussion on #4465.

For now, do people like this approach? Broadly:

• imagerepo.json stores a list of known good files for each test, and the known hashes for each file.
• If the hashes shift without the images changing, imagehash_update.py can be used to update imagerepo.json (which holds all historically known hashes as well as the current one).
• If you've broken an image test, or matplotlib have updated and broken it, then you follow the old procedure for updating those (using idiff.py).

I envisage that you won't normally want to update imagehashes from dependency changes on a feature branch, and we'll instead do a quick fix branch then everyone can rebase to fix their graphical tests. We could put the functionality into test-iris-imagehash instead of Iris if that works better, though that would mean keeping imagerepo.json there instead so maybe not?

To do if people like the broad idea:

• Update imagerepo.json to include the alternative names that images have in the test-iris-imagehash repo currently (v3 vs v4)
• Finish changing idiff.py
• Test it
• Rename anything that wants renaming (e.g. the other repo, imagerepo.json)
• Pull out shared functionality of idiff.py and imagehash_update.py or make them different modes of the same script
• Update imagehash_update.py to use most sensible defaults, potentially pull images off the web etc.
• Restructure test-iris-imagehash
• Update documentation
• What's new

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