Add JPEG XL Open/Read support via libjxl

[olokelo]

Helps #4247

This PR enables opening and reading JPEG XL images and animations.
Supported image modes are: RGB, RGBA, RGBa, L, LA, La.
A relatively recent libjxl version is needed to compile Pillow with libjxl support.
The main changes are the addition of _jxl.c and JxlImagePlugin.py.
I'm also the author of jxlpy so this PR was influenced by the work of contributors there. This PR is also largely based on WebPImagePlugin.py which had similar implementation.

Why?
JPEG XL has recently seen increased adoption especially in Apple ecosystem. A lot of users are requesting Pillow support for JPEG XL as their products use Pillow and need to be able to handle jxl files.

I'm open to suggestions and comments. I understand such change would need a lot of testing and probably changes. After all Pillow would need to become somewhat dependent on libjxl. Creating documentation will not be a big problem however I decided to wait for feedback from Pillow core developers.

[brunoais]

May you please commit the images as LFS?

[olokelo]

Do you mean those .jxl files under Tests/images I've committed? I'm not really sure how to do that.

[brunoais]

The explanation is outlined here: https://docs.github.com/en/repositories/working-with-files/managing-large-files/configuring-git-large-file-storage
Nevermind. I just noticed they are not putting the test images as lfs:

They already setup lfs and they only use for other larger stuff. Let it stay as you did.

[olokelo]

Mac OS builds were failing because clang complained about goto labels being declared before variables in scope.
I also merged @radarhere's commits that remove the jxl feature which was causing troubles before.
Now it should be fine. Almost all checks pass except codecov.

[hugovk]

How much bigger does this make the wheels?

We need to keep in mind increased wheel size effects everyone, even if they don't use JXL (not the case with opt-in plugins).

When we added AVIF, we initially had problems with the wheels being too big: #8856

[radarhere]

It does increase the size significantly. The current total size of our wheels is 531.5mb. In this PR, it is 730.2mb.

[hugovk]

It does increase the size significantly. The current total size of our wheels is 531.5mb. In this PR, it is 730.2mb.

This seems like too much. I know we have fewer wheels right now, but it will soon increase for 3.15+3.15t, and a ~37% increase is a lot. And it will be paid by everyone, including those not using JPEG XL.

I think we should consider some of the suggestions I made at #9101 (comment).

[h-vetinari]

I think we should consider some of the suggestions I made at #9101 (comment).

I think that's a very worthwhile initiative, though IMO that should be pursued independently; IOW, I wouldn't like to see jpegxl support held up over this.

[aclark4life]

I think that's a very worthwhile initiative, though IMO that should be pursued independently; IOW, I wouldn't like to see jpegxl support held up over this.

We can try to decide on and implement some alternative to "including in the wheel" prior to the next release.

[hugovk]

I think that's a very worthwhile initiative, though IMO that should be pursued independently; IOW, I wouldn't like to see jpegxl support held up over this.

The thing is, the current size increase is a blocker for me.