Function permMod12

crawl-ref/source/perlin.cc:98–101

        138,236,205,93,222,114,67,29,24,72,243,141,128,195,78,66,215,61,156,180};

    static IMMUTABLE uint8_t permMod12(const uint32_t x)
    {
        return perm[x] % 12;
    }

    // Skewing and unskewing factors for 2, 3, and 4 dimensions
    static const double F2 = 0.5 * (sqrt(3.0) - 1.0);