public synchronized void addLayerToGroup(final Layer layer, final int groupIndex) {
        mMapLayers.addToGroup(layer, groupIndex);
    }
call this instead of VTM.layers().add so that we can keep track of layers added by the tile provider
main/src/main/java/cgeo/geocaching/unifiedmap/mapsforgevtm/MapsforgeVtmFragment.java