draw by layer, not by tile

This is needed for correctly compositing raster imagery into the map

fixes #122
fixes #157

src/map/vector_tile_data.cpp

@@ -40,4 +40,4 @@ void VectorTileData::render(Painter &painter, const LayerDescription& layer_desc
         assert(databucket_it->second);
         databucket_it->second->render(painter, layer_desc.name, id);
     }
-}
+}