Fix missing initialization of Map::Impl::sceneLoadTasks (#1710)
Fix missing initialization of Map::Impl::sceneLoadTasks (#1710)

* Fix missing initialization of std::atomic<int32_t> Map::Impl::sceneLoadTasks

* use initializer at declaration site

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