Motivation for this change

According to the source of mautrix-telegram, this binary (and indirectly rlottie as a library) is required to convert animated telegram stickers to a different format. As mautrix-telegram has a module (#63589 ) in unstable, it would make sense to care for this feature.

I'm unsure whether the derivation for rlottie is written correctly. The cmake script seems a bit broken, as most variables are not used. It also has a very mixed license usage (from MIT over custom unfree licenses), so I was unsure whether I should list all of them, and went the most restrictive path.
LottieConverter has its license written in the code file.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.