megapixels: 0.14.0 -> 0.15.0

[OPNA2608]

Motivation for this change

https://git.sr.ht/~martijnbraam/megapixels/refs/0.15.0

Megapixels 0.15.0

• Support both imagemagick and graphicsmagick in the default postprocess script
• Add the QR code support from benjamin-schaaf

QR detection & reading works, though copying the link or opening it in a browser doesn't on my non-NixOS OS.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS (x86_64 & aarch64)
  • 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)
  532479168 -> 567197688 (573583216 w/ imagemagick)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.