builder-main: Allow specifying appstream-compose image format

[bbhtt]

This additionally ensures the default format stays PNG consistent with
pre-1.2.0 behaviour and to preserve backwards compat while allowing
JXL if the required AppStream version (>=1.2.0) is available.

Related: #777
Related: #776
Closes: #776
Closes: #777

Can't find CI with 1.2.0 yet so haven't added tests...

[bbhtt]

Yeah will have to add 1.2.0 CI later on. Building 1.2.0 in CI is complicated. It needs a newer versions of a bunch of dependencies gcc meson and some new dependencies too which aren't in the 22.04/24.04 repos yet.

I'll probably switch to newer Ubuntu or Fedora once they have it.

[bbhtt]

I added a tentative test for JXL in the last commit. Needs to be verified it is working.

[smcv]

I'll probably switch to newer Ubuntu or Fedora once they have it

Ubuntu LTS is unlikely to pick up a new appstream package in anything less than a new major release (the next one will be 28.04 in April 2028). To test against the latest software, you'll have to use a development or rolling-release distribution like Debian unstable, Fedora rawhide or Arch, or at least a short-term-support release like Fedora.

[smcv]

The changes generally good to me, but the tests are failing on Debian unstable. I'll see if I can get more useful debug info out of them.

Are you intending to merge this into main as well as 1.4.x, when it works? (I ask so that I can try to rebase #778 to get useful diagnosis.)

[bbhtt]

Yes I do intend to merge on both branches. If you can get the info on why the tests are failing it would be great.

you'll have to use a development or rolling-release distribution like Debian unstable,

Yep, but only Debian unstable has it right now and it is a bit of a pain to use in CI as it's not a full repository. The unstable distros/branches are also a bit too rolling for CI to my taste.

I intend on switching to Fedora when they'll have it.

[smcv]

not a full repository

Er, it is? Debian unstable is a complete, self-contained distribution just like stable.

Perhaps you're thinking of experimental, which is an overlay over unstable, the same way that stable-backports is an overlay over stable?

[bbhtt]

Ah right I confused it with experimental

[smcv]

If you can get the info on why the tests are failing it would be great

Half of it was a timeout, fixed (or at least worked around) by #780. I'm still struggling with the shell scripts to figure out why test-appstream.sh is failing.

[bbhtt]

Ok thanks. I'll try to figure it out if you don't have the time.

[smcv]

Now works (when rebased on #780), thanks!

[bbhtt]

Ok thanks. I'll merge this and merge your test improvement PR then cut a release.

I'll open an issue to track the missing CI part.