function targetTo(out, eye, target, up) {
  var eyex = eye[0],
      eyey = eye[1],
      eyez = eye[2],
      upx = up[0],
      upy = up[1],
      upz = up[2];

  var z0 = eyex - target[0],
      z1 = eyey - target[1],
      z2 = eyez - target[2];

  var len = z0 * z0 + z1 * z1 + z2 * z2;
  if (len > 0) {
    len = 1 / Math.sqrt(len);
    z0 *= len;
    z1 *= len;
    z2 *= len;
  }

  var x0 = upy * z2 - upz * z1,
      x1 = upz * z0 - upx * z2,
      x2 = upx * z1 - upy * z0;

  out[0] = x0;
  out[1] = x1;
  out[2] = x2;
  out[3] = 0;
  out[4] = z1 * x2 - z2 * x1;
  out[5] = z2 * x0 - z0 * x2;
  out[6] = z0 * x1 - z1 * x0;
  out[7] = 0;
  out[8] = z0;
  out[9] = z1;
  out[10] = z2;
  out[11] = 0;
  out[12] = eyex;
  out[13] = eyey;
  out[14] = eyez;
  out[15] = 1;
  return out;
}
Generates a matrix that makes something look at something else.

@param {mat4} out mat4 frustum matrix will be written into
@param {vec3} eye Position of the viewer
@param {vec3} center Point the viewer is looking at
@param {vec3} up vec3 pointing up
@returns {mat4} out