Method seek

tests/fl/fx/video.cpp:66–71

    size_t tell() override { return mPos; }
    const char *path() const override { return "fake"; }
    bool seek(size_t pos, fl::seek_dir dir) override {
        if (dir == fl::seek_dir::beg) { this->mPos = pos; }
        else if (dir == fl::seek_dir::cur) { this->mPos += pos; }
        else { this->mPos = data.size() + pos; }
        return true;
    }
    using fl::filebuf::seek; // single-arg overload
    void close() override {}
    bool is_eof() const override { return mPos >= data.size(); }