Add thread safety to WIC initialization using call_once #807
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Technically the singleton DefaultWicAdapter instance was not thread-safe when creating the WIC factory. Similarly, the DefaultCanvasImageAdapter was not thread-safe when storing a copy of the WicAdapter instance. In the second case in practice it probably was thread-safe anyway assuming setting a pointer is atomic. In the first case it would probably have been very difficult to hit a serious issue, although potentially it could have leaked the factory object and potentially returned a different instance on different calls.
This pull request fixes that by using std::call_once.