function frob(a) {
    return Math.hypot(a[0], a[1], a[2], a[3], a[4], a[5], a[6], a[7], a[8], a[9], a[10], a[11], a[12], a[13], a[14], a[15]);
  }
Returns Frobenius norm of a mat4

@param {ReadonlyMat4} a the matrix to calculate Frobenius norm of
@returns {Number} Frobenius norm