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 {vec2} a vector to normalize
@returns {vec2} out