message: Fix typo in from_json for vec4f
diff: @@ -290,7 +290,7 @@ inline void from_json(const JSON &j, vec4f &v)
  j.at(0).get_to(v.x);
  j.at(1).get_to(v.y);
  j.at(2).get_to(v.z);
  - j.at(2).get_to(v.w);
  + j.at(3).get_to(v.w);
  }
  inline void to_json(JSON &j, const LinearSpace3f &ls)
  