Direct2D: Reinstate multithreading support for D2D factory

Partially reverts a change made in
362a1cc070.

The factory needs to be multithreading-enabled in order to support e.g.
drawing to the screen on the main thread and drawing to an Image on a
background thread, even when no resources other than the graphics device
are shared between the two threads.

modules/juce_graphics/native/juce_DirectX_windows.h

@@ -269,7 +269,7 @@ private:
         options.debugLevel = D2D1_DEBUG_LEVEL_NONE;
         JUCE_BEGIN_IGNORE_WARNINGS_GCC_LIKE ("-Wlanguage-extension-token")
         ComSmartPtr<ID2D1Factory2> result;
-        auto hr = D2D1CreateFactory (D2D1_FACTORY_TYPE_SINGLE_THREADED,
+        auto hr = D2D1CreateFactory (D2D1_FACTORY_TYPE_MULTI_THREADED,
                                      __uuidof (ID2D1Factory2),
                                      &options,
                                      (void**) result.resetAndGetPointerAddress());