Fix IMBackend#compare on ImageMagick 7.1.1-44

[jackwilsdon]

Description

ImageMagick 7.1.1-44 now outputs phash result in a different format (as of ImageMagick/ImageMagick@d85a758):

$ magick --version | head -n1
Version: ImageMagick 7.1.1-43 Q16-HDRI x86_64 afd817ca6:20241222 https://imagemagick.org
$ compare -metric PHASH a.jpg b.jpg NULL:
649.922

$ magick --version | head -n1
Version: ImageMagick 7.1.1-44 Q16-HDRI x86_64 f1c26bed0:20250222 https://imagemagick.org
$ compare -metric PHASH a.jpg b.jpg NULL:
4.25933e+07 (649.931)

Fixes #5527.

To Do

• Documentation. (If you've added a new command-line flag, for example, find the appropriate page under docs/ to describe it.)
• Changelog. (Add an entry to docs/changelog.rst to the bottom of one of the lists near the top of the document.)
• Tests. (Very much encouraged but not strictly required.)

[doronbehar]

We can confirm this fixes the issue on NixOS, is there any objection for this to getting merged?

[jackwilsdon]

I'm happy to merge this, I've also tested it on NixOS 👍 not sure when the next release is but I guess nixpkgs can pull in this patch for now.

[doronbehar]

Thanks :). Merging this gives us confident that the patch link will stay stable and the hash of it stable as well, because the PR's contents don't change.