Reset mIsReleased before starting threads

To fix TSAN warning:

WARNING: ThreadSanitizer: data race (pid=68597)
  Write of size 1 at 0x7b3800079234 by main thread:
    #0 SceneUtil::WorkQueue::start(unsigned long) /home/elsid/dev/openmw/components/sceneutil/workqueue.cpp:51 (openmw+0x10daa10)
    openscenegraph#1 SceneUtil::WorkQueue::WorkQueue(unsigned long) /home/elsid/dev/openmw/components/sceneutil/workqueue.cpp:39 (openmw+0x10dad97)
    openscenegraph#2 OMW::Engine::prepareEngine(Settings::Manager&) /home/elsid/dev/openmw/apps/openmw/engine.cpp:700 (openmw+0xf7cb5a)
    openscenegraph#3 OMW::Engine::go() /home/elsid/dev/openmw/apps/openmw/engine.cpp:949 (openmw+0xf82688)
    openscenegraph#4 runApplication(int, char**) /home/elsid/dev/openmw/apps/openmw/main.cpp:316 (openmw+0xf62611)
    openscenegraph#5 wrapApplication(int (*)(int, char**), int, char**, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&) /home/elsid/dev/openmw/components/debug/debugging.cpp:205 (openmw+0x125df1c)
    openscenegraph#6 main /home/elsid/dev/openmw/apps/openmw/main.cpp:328 (openmw+0x596323)

  Previous read of size 1 at 0x7b3800079234 by thread T10 (mutexes: write M19275778865205896):
    #0 SceneUtil::WorkQueue::removeWorkItem() /home/elsid/dev/openmw/components/sceneutil/workqueue.cpp:86 (openmw+0x10d9e51)
    openscenegraph#1 SceneUtil::WorkThread::run() /home/elsid/dev/openmw/components/sceneutil/workqueue.cpp:127 (openmw+0x10da52a)
    openscenegraph#2 operator() /home/elsid/dev/openmw/components/sceneutil/workqueue.cpp:114 (openmw+0x10da664)
    openscenegraph#3 __invoke_impl<void, SceneUtil::WorkThread::WorkThread(SceneUtil::WorkQueue&)::<lambda()> > /usr/include/c++/11.1.0/bits/invoke.h:61 (openmw+0x10da664)
    openscenegraph#4 __invoke<SceneUtil::WorkThread::WorkThread(SceneUtil::WorkQueue&)::<lambda()> > /usr/include/c++/11.1.0/bits/invoke.h:96 (openmw+0x10da664)
    openscenegraph#5 _M_invoke<0> /usr/include/c++/11.1.0/bits/std_thread.h:253 (openmw+0x10da664)
    openscenegraph#6 operator() /usr/include/c++/11.1.0/bits/std_thread.h:260 (openmw+0x10da664)
    openscenegraph#7 _M_run /usr/include/c++/11.1.0/bits/std_thread.h:211 (openmw+0x10da664)
    openscenegraph#8 execute_native_thread_routine /build/gcc/src/gcc/libstdc++-v3/src/c++11/thread.cc:82 (libstdc++.so.6+0xd33c3)

  Location is heap block of size 216 at 0x7b3800079220 allocated by main thread:
    #0 operator new(unsigned long) /build/gcc/src/gcc/libsanitizer/tsan/tsan_new_delete.cpp:64 (libtsan.so.0+0x91824)
    openscenegraph#1 OMW::Engine::prepareEngine(Settings::Manager&) /home/elsid/dev/openmw/apps/openmw/engine.cpp:700 (openmw+0xf7cb4c)
    openscenegraph#2 OMW::Engine::go() /home/elsid/dev/openmw/apps/openmw/engine.cpp:949 (openmw+0xf82688)
    openscenegraph#3 runApplication(int, char**) /home/elsid/dev/openmw/apps/openmw/main.cpp:316 (openmw+0xf62611)
    openscenegraph#4 wrapApplication(int (*)(int, char**), int, char**, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&) /home/elsid/dev/openmw/components/debug/debugging.cpp:205 (openmw+0x125df1c)
    openscenegraph#5 main /home/elsid/dev/openmw/apps/openmw/main.cpp:328 (openmw+0x596323)

  Mutex M19275778865205896 is already destroyed.

  Thread T10 (tid=68609, running) created by main thread at:
    #0 pthread_create /build/gcc/src/gcc/libsanitizer/tsan/tsan_interceptors_posix.cpp:969 (libtsan.so.0+0x61c3a)
    openscenegraph#1 std::thread::_M_start_thread(std::unique_ptr<std::thread::_State, std::default_delete<std::thread::_State> >, void (*)()) /build/gcc/src/gcc-build/x86_64-pc-linux-gnu/libstdc++-v3/include/x86_64-pc-linux-gnu/bits/gthr-default.h:663 (libstdc++.so.6+0xd36aa)
    openscenegraph#2 std::_MakeUniq<SceneUtil::WorkThread>::__single_object std::make_unique<SceneUtil::WorkThread, SceneUtil::WorkQueue&>(SceneUtil::WorkQueue&) /usr/include/c++/11.1.0/bits/unique_ptr.h:962 (openmw+0x10da987)
    openscenegraph#3 SceneUtil::WorkQueue::start(unsigned long) /home/elsid/dev/openmw/components/sceneutil/workqueue.cpp:50 (openmw+0x10da987)
    openscenegraph#4 SceneUtil::WorkQueue::WorkQueue(unsigned long) /home/elsid/dev/openmw/components/sceneutil/workqueue.cpp:39 (openmw+0x10dad97)
    openscenegraph#5 OMW::Engine::prepareEngine(Settings::Manager&) /home/elsid/dev/openmw/apps/openmw/engine.cpp:700 (openmw+0xf7cb5a)
    openscenegraph#6 OMW::Engine::go() /home/elsid/dev/openmw/apps/openmw/engine.cpp:949 (openmw+0xf82688)
    openscenegraph#7 runApplication(int, char**) /home/elsid/dev/openmw/apps/openmw/main.cpp:316 (openmw+0xf62611)
    openscenegraph#8 wrapApplication(int (*)(int, char**), int, char**, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&) /home/elsid/dev/openmw/components/debug/debugging.cpp:205 (openmw+0x125df1c)
    openscenegraph#9 main /home/elsid/dev/openmw/apps/openmw/main.cpp:328 (openmw+0x596323)

components/sceneutil/workqueue.cpp

@@ -46,9 +46,12 @@ WorkQueue::~WorkQueue()
 
 void WorkQueue::start(std::size_t workerThreads)
 {
+    {
+        const std::lock_guard lock(mMutex);
+        mIsReleased = false;
+    }
     while (mThreads.size() < workerThreads)
         mThreads.emplace_back(std::make_unique<WorkThread>(*this));
-    mIsReleased = false;
 }
 
 void WorkQueue::stop()