ffmpeg: build with celt only in -full

[LordGrimmauld]

Celt codec is effectively unused and superseeded by libopus. It makes little sense to support it in the minimal build.

If anyone still uses ffmpeg with celt, it is available in -full for now.

ffmpeg still has a transient dep on celt via libjack2, my goal is to drop that too.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 25.05 Release Notes (or backporting 24.11 and 25.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

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[Atemu]

Celt codec is effectively unused

[citation needed]

You and I would choose Opus over Celt every day but proprietary/legacy apps exist. Have you verified somehow that no valuable package depends on ffmpeg's celt support?

[LordGrimmauld]

This is extremely hard to verify. If such an app does exist, it can still get its celt support via ffmpeg-full if actually needed. Consensus on the ffmpeg IRC was: there are no known users. In nixkgs, there is few direct users of celt: mumble (vendors its own celt), jack1 (effectively unused too), baresip (pins its own celt). Mumble and baresip were the two that came up, and both are independent from ffmpeg. Other distros, such as fedora, also do not package their ffmpeg with celt - it is questionable any project making use of ffmpeg could reasonably assume celt to be enabled in the first place.

If by some dumb chance something does use celt, it can use it via ffmpeg-full if absolutely necessary. But i am willing to bet quite a bit that is not the case.

Though waiting for maintainer feedback before a merge here would have probably been appropriate...