Windows: Call ReleaseDC for the private's displayContext

Change-Id: Ib5ee1bbe9037ceb13562eadb754c2a5f095b7f87
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>

src/plugins/platforms/windows/qwindowscontext.cpp

@@ -317,6 +317,8 @@ QWindowsContext::~QWindowsContext()
         OleUninitialize();
 
     d->m_screenManager.clearScreens(); // Order: Potentially calls back to the windows.
+    if (d->m_displayContext)
+        ReleaseDC(nullptr, d->m_displayContext);
     m_instance = nullptr;
 }