Windows: Add helper function to initialize COM per thread

This adapts a pattern first introduced for qtmultimedia (7398c5daa9).

Pick-to: 6.9 6.9.0
Change-Id: I4682fcf49fe074c55b4a70a1e1ec659b33b31bbb
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/kernel/qfunctions_win.cpp

@@ -33,6 +33,23 @@ QComHelper::~QComHelper()
         CoUninitialize();
 }
 
+/*!
+    \internal
+    Make sure the COM library is is initialized on current thread.
+
+    Initializes COM as a single-threaded apartment on this thread and
+    ensures that CoUninitialize will be called on the same thread when
+    the thread exits. Note that the last call to CoUninitialize on the
+    main thread will always be made during destruction of static
+    variables at process exit.
+
+    https://docs.microsoft.com/en-us/windows/apps/desktop/modernize/modernize-packaged-apps
+*/
+void qt_win_ensureComInitializedOnThisThread()
+{
+    static thread_local QComHelper s_comHelper;
+}
+
 /*!
     \internal
     Checks if the application has a \e{package identity}

src/corelib/kernel/qfunctions_win_p.h

@@ -42,6 +42,8 @@ private:
     DWORD m_threadId{ GetCurrentThreadId() };
 };
 
+Q_CORE_EXPORT void qt_win_ensureComInitializedOnThisThread();
+
 Q_CORE_EXPORT bool qt_win_hasPackageIdentity();
 
 QT_END_NAMESPACE