Fix the tst_QOpenGL::sharedResourceCleanup() test case in Windows

QWindowsGLContext was returning always false for isSharing() query,
as it was falling back to default implementation.

Task-number: QTBUG-24192
Change-Id: I884d559d69545ef85a0d07d7287bc50ce459c1bd
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@nokia.com>

src/plugins/platforms/windows/qwindowsglcontext.h

@@ -152,6 +152,7 @@ public:
     explicit QWindowsGLContext(const QOpenGLStaticContextPtr &staticContext,
                                QOpenGLContext *context);
     virtual ~QWindowsGLContext();
+    bool isSharing() const                { return m_context->shareHandle(); }
     bool isValid() const                  { return m_renderingContext; }
     virtual QSurfaceFormat format() const { return m_obtainedFormat; }