Fix macOS CI build break from removed std::result_of in vendored Eigen

[Copilot]

macos-latest-build fails on current macOS toolchains because vendored Eigen still references std::result_of, which is not available in modern C++/libc++ configurations. This PR updates the affected Eigen type-trait path to select a C++17-compatible alternative while preserving backward compatibility.

• Root-cause area

  • filmulator-gui/core/nlmeans/eigen/Eigen/src/Core/util/Meta.h
  • Failure originated in Eigen’s internal result_of wrapper (no template named 'result_of' in namespace 'std').

• Targeted compatibility fix

  • Switch to std::invoke_result for C++17+ compilation modes.
  • Keep std::result_of for older language modes.
  • Include MSVC-safe language-mode detection (_MSVC_LANG) alongside __cplusplus.

• Scope

  • Single-file, localized change in vendored Eigen metaprogramming layer used by nlmeans codepaths.

template<typename T> struct result_of {
#if (defined(__cplusplus) && __cplusplus >= 201703L) || (defined(_MSVC_LANG) && _MSVC_LANG >= 201703L)
  typedef typename std::invoke_result<T>::type type1;
#else
  typedef typename std::result_of<T>::type type1;
#endif
  typedef typename remove_all<type1>::type type;
};