static void quarterRound(int[] x, int a, int b, int c, int d) {
    x[a] += x[b];
    x[d] = rotateLeft(x[d] ^ x[a], 16);
    x[c] += x[d];
    x[b] = rotateLeft(x[b] ^ x[c], 12);
    x[a] += x[b];
    x[d] = rotateLeft(x[d] ^ x[a], 8);
    x[c] += x[d];
    x[b] = rotateLeft(x[b] ^ x[c], 7);
  }
Computes the ChaCha quarter round as described in
https://datatracker.ietf.org/doc/html/rfc7539#section-2.1.
java_src/src/main/java/com/google/crypto/tink/aead/internal/ChaCha20Util.java