Fix race condition in QOpenGLMultiGroupSharedResource

In QOpenGLMultiGroupSharedResource::value(), the m_resources memeber of
opengl context group will be checked and may be inserted a new value. This
function may be called from different threads with the same opengl context
group object: Think about that OpenGL context A and B are shared, they
live in different thread, and in each thread we call
QOpenGLContext::functions() to the context, that may cause the race I
mentioned above. So I put a QMutexLocker to prevent race condition.

Change-Id: I101a28f46b2af72b3094a9945880efeb3563482a
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>

src/gui/kernel/qopenglcontext_p.h

@@ -171,6 +171,7 @@ public:
     template <typename T>
     T *value(QOpenGLContext *context) {
         QOpenGLContextGroup *group = context->shareGroup();
+        QMutexLocker locker(&group->d_func()->m_mutex);
         T *resource = static_cast<T *>(group->d_func()->m_resources.value(this, 0));
         if (!resource) {
             resource = new T(context);