Motivation for this change

Security release fixing numerous issues. See http://www.graphicsmagick.org/NEWS.html#june-15-2019 - presumably too many to assign CVEs to.

Darwin requires an extra patch, which I've included as a cut-down version of https://sourceforge.net/p/graphicsmagick/code/ci/f30492f40f78d867b43422215057dd21de4ba447 - we weren't able to find an url-addressable endpoint in sourceforge that would give us a raw .patch, despite the heroic efforts of @samueldr . Without this, use of pngs is liable to raise a SIGABRT and specifically this caused dosbox to fail to build on darwin.

As far as backporting goes - though I'm usually in favour of patching, I do imagine we're going to have to go with a full version bump for 19.03 - you just have to look at the sheer number of fixes in the release notes to see it wouldn't be practical to patch.

nox-review build failures are the same as without the bump, tested on macos 10.13 and non-nixos linux x86_64.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• 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 nix-review --run "nix-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.