Tell ANGLE to enable multithread protection.

This is done on initialization of the Web Engine as this should
happen before any contexts and Direct 3D devices are created.

It makes D3D9 create a device with the D3DCREATE_MULTITHREADED
flag and makes D3D11 enable multithread protection using the
ID3D10Multithread interface.

Depends on the appropriate counterpart in QtANGLE.

Change-Id: I8204de2f8ebe993273ff9f11af55caacb1290e4e
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@digia.com>

src/webengine/api/qtwebengineglobal.cpp

@@ -74,6 +74,10 @@ void initialize()
     if (shareContext)
         return;
 
+#ifdef Q_OS_WIN32
+    qputenv("QT_D3DCREATE_MULTITHREADED", "1");
+#endif
+
     shareContext = new QOpenGLContext;
     shareContext->create();
     qAddPostRoutine(deleteShareContext);