[Qt] Use QThreadStorage for thread local

Portable.

platform/qt/qt.cmake

@@ -37,7 +37,7 @@ set(MBGL_QT_FILES
     PRIVATE platform/default/mbgl/util/default_thread_pool.hpp
 
     # Thread
-    PRIVATE platform/default/thread_local.cpp
+    PRIVATE platform/qt/src/thread_local.cpp
 
     # Platform integration
     PRIVATE platform/qt/src/async_task.cpp

platform/qt/src/thread_local.cpp

@@ -0,0 +1,44 @@
+#include <mbgl/util/thread_local.hpp>
+
+#include <mbgl/util/run_loop.hpp>
+#include <mbgl/map/backend_scope.hpp>
+
+#include <array>
+
+#include <QThreadStorage>
+
+namespace mbgl {
+namespace util {
+
+template <class T>
+class ThreadLocal<T>::Impl {
+public:
+    QThreadStorage<std::array<T*, 1>> local;
+};
+
+template <class T>
+ThreadLocal<T>::ThreadLocal() : impl(std::make_unique<Impl>()) {
+    set(nullptr);
+}
+
+template <class T>
+ThreadLocal<T>::~ThreadLocal() {
+    delete get();
+}
+
+template <class T>
+T* ThreadLocal<T>::get() {
+    return impl->local.localData()[0];
+}
+
+template <class T>
+void ThreadLocal<T>::set(T* ptr) {
+   impl->local.localData()[0] = ptr;
+}
+
+template class ThreadLocal<RunLoop>;
+template class ThreadLocal<BackendScope>;
+template class ThreadLocal<int>; // For unit tests
+
+} // namespace util
+} // namespace mbgl