QtWebEngineCore do not clean the shared context

When you create a QApplication, which have QWebEngine, a shared context is created.
But when the application is destroyed, the shared context pointer is not reset to nullptr.

So, in a UnitTest context, if we create a QApplication and QWebEngine by TestCase, it will crash on the second test case, because a deleted shared context will still be reference by global_share_context (in qopenglcontext.cpp).

The problem seems due to the call to deleteShareContext() in qtwebenginecoreglobal.cpp. This function do not recall qt_gl_set_global_share_context() with nullptr in order to clear the pointer of the deleted shared context.