Method benchmark

tutorials/verify/verify.cpp:5742–5750

    virtual void render_block(size_t i, size_t n) = 0;

    float benchmark(VerifyApplication* state)
    {
      double t0 = getSeconds();
      parallel_for((size_t)N/dN, [&](size_t i) {
          render_block(i*dN, dN);
      });
      double t1 = getSeconds();
      return 1E-6f * (float)(N)/float(t1-t0);
    }

    virtual void cleanup(VerifyApplication* state) 
    {