public synchronized void addLayer(final int index, final Layer layer) {
        layers.add(layer);
        mMapLayers.addToGroup(layer, index);
    }
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