title: render option in Layer
body: <p>Anyone know what the use of the <a href="https://github.com/openlayers/openlayers/blob/deaa0ccc2ae670516a2ef1ff52fab129fd669600/src/ol/layer/Layer.js#L150" rel="nofollow noreferrer">render option</a>
  that can be passed to Layer constructor?</p>
  <p>it's seem that is not effect on anything or
  that the render function is missing the prefix</p>
  <p><code>if(this.render) return this.render(frameState, target)</code></p>