libjxl: update to 0.12

[yoshiyoshyosh]

Testing the changes

• I tested the changes in this PR: YES
  • I've been running my own master branch libjxl and periodically updating it in the past several months. I haven't thoroughly tested every dependency, but they do all build. most of the changes are fixes and optimizations.

Local build testing

• I built this PR locally for my native architecture, x86_64-glibc

[yoshiyoshyosh]

cc @Johnnynator: the kf6-kimageformats package tests fail on kimageformats-write-hej2 due to the configuration not supporting hej2. I notice that kimageformats-read-hej2 test was already excluded, and that one fails for the same reason, so I'm excluding the write-hej2 test as well.