Flatten layers before synchronizeSingle
Flatten layers before synchronizeSingle

Move handling the layer group out of synchronizeSingle.

epointal/ol3-cesium,epointal/ol3-cesium,thhomas/ol3-cesium,oterral/ol3-cesium,bartvde/ol3-cesium,gberaudo/ol3-cesium,openlayers/ol3-cesium,gberaudo/ol3-cesium,thhomas/ol3-cesium,GistdaDev/ol3-cesium,openlayers/ol3-cesium,thhomas/ol3-cesium,openlayers/ol-cesium,GistdaDev/ol3-cesium,oterral/ol3-cesium,openlayers/ol-cesium,bartvde/ol3-cesium