HEIF prototype via ffmpeg

[fstanis]

#964 (comment) for context

[CyberTimon]

@fstanis It seems like the builds fail because you force C23 (.clang_args(["-x", "c", "-std=c23"])) in your ffheif repository. Is there a reason for this or can you edit it to use a widely supported one?

[fstanis]

Right, that's just a remnant of some older C++ prototype - I changed it so that it uses --std=c17 and rebuilt.