Stop engine work queue before destructing environment

To avoid access to null and dangling pointers from active work items on
quitting.

apps/openmw/engine.cpp

@@ -407,6 +407,8 @@ OMW::Engine::Engine(Files::ConfigurationManager& configurationManager)
 
 OMW::Engine::~Engine()
 {
+    mWorkQueue->stop();
+
     mEnvironment.cleanup();
 
     delete mScriptContext;

components/sceneutil/workqueue.cpp

@@ -33,14 +33,25 @@ bool WorkItem::isDone() const
     return mDone;
 }
 
-WorkQueue::WorkQueue(int workerThreads)
+WorkQueue::WorkQueue(std::size_t workerThreads)
     : mIsReleased(false)
 {
-    for (int i=0; i<workerThreads; ++i)
+    start(workerThreads);
-        mThreads.emplace_back(std::make_unique<WorkThread>(*this));
 }
 
 WorkQueue::~WorkQueue()
+{
+    stop();
+}
+
+void WorkQueue::start(std::size_t workerThreads)
+{
+    while (mThreads.size() < workerThreads)
+        mThreads.emplace_back(std::make_unique<WorkThread>(*this));
+    mIsReleased = false;
+}
+
+void WorkQueue::stop()
 {
     {
         std::unique_lock<std::mutex> lock(mMutex);

components/sceneutil/workqueue.hpp

@@ -44,9 +44,13 @@ namespace SceneUtil
     class WorkQueue : public osg::Referenced
     {
     public:
-        WorkQueue(int numWorkerThreads=1);
+        WorkQueue(std::size_t workerThreads);
         ~WorkQueue();
 
+        void start(std::size_t workerThreads);
+
+        void stop();
+
         /// Add a new work item to the back of the queue.
         /// @par The work item's waitTillDone() method may be used by the caller to wait until the work is complete.
         /// @param front If true, add item to the front of the queue. If false (default), add to the back.