function normalize(out, a) {
    var x = a[0],
        y = a[1];
    var len = x * x + y * y;

    if (len > 0) {
      //TODO: evaluate use of glm_invsqrt here?
      len = 1 / Math.sqrt(len);
    }

    out[0] = a[0] * len;
    out[1] = a[1] * len;
    return out;
  }
Normalize a vec2

@param {vec2} out the receiving vector
@param {ReadonlyVec2} a vector to normalize
@returns {vec2} out