ffmpeg_4: build with videotoolbox on darwin

[ryan4729]

Motivation for this change

Build ffmpeg_4 with Apple's VideoToolbox video acceleration framework. The motivation for this change was to be able to use hardware accelerated playback on the mpv video player on macOS. After this change, you can see that mpv <video_file> --msg-level=all=v --hwdec=videotoolbox provides hardware accelerated playback.

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 nox --run "nox-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)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.

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[veprbl]

I tested this and got CPU load reduced by a factor of 3 or more. Definitely useful!