Fix toggle behaviour of the Select interaction

Because features on overlays are skipped and therefore not seen by forEachFeatureAtPixel, it is not possible to toggle the selection of a skipped feature. By making the skipped features for forEachFeatureAtPixel configurable at the map renderer level, the Select interaction can do its hit detection without excluding the skipped feature of its selection overlay.

The issue was reported on the [mailing list](https://groups.google.com/d/msg/ol3-dev/_C0XLSBIBR0/KnvDm3rXBl4J), and this is a solution that works independently of #1591.