Fix: move TileData out of NetworkDataSource urlcallback
Fix: move TileData out of NetworkDataSource urlcallback

- Otherwise a ready Tile may be destructed when getting back to
  UrlClient thread after the tile got processed by TileWoker.
  Its Scene may already disposed by then - Text mesh destruction
  will callback to Style::fontContext.releaseAtlas()
- Another approach to fix this would be keep hold of Scene in urlcallback
  (instead of just the TileSource) - Already done in an upcoming PR

tangrams/tangram-es,tangrams/tangram-es,tangrams/tangram-es,tangrams/tangram-es,tangrams/tangram-es,tangrams/tangram-es,tangrams/tangram-es