Wait for initialization when adding assets at runtime. Correctly destroy the RenderableGalaxy shader when deinitializing (closes #1601)

include/openspace/util/threadpool.h

@@ -56,6 +56,8 @@ public:
     void enqueue(std::function<void()> f);
     void clearTasks();
 
+    bool hasOutstandingTasks() const;
+
 private:
     friend class Worker;