[vector-api] Fix forEachFeatureAtPixel bug

This PR fixes a bug in the hit detection code. The bug triggers when the map includes layer groups. An [assertion](https://github.com/openlayers/ol3/blob/vector-api/src/ol/renderer/canvas/canvasmaprenderer.js#L74) fails because getLayerRenderer attemps to create a layer renderer for a layer group.

@oterral, can you please verify that this fixes the bug for you?