tst_qglthreads::swapInThread(): Acquire context before resizing widget

Otherwise the QGLWidget's context may be owned by and used by the swap
thread when the resize happens on the main thread.

Change-Id: Id415747572e6b988924618b1df0efdee5fcb1805
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>

tests/auto/opengl/qglthreads/tst_qglthreads.cpp

@@ -131,6 +131,13 @@ public:
         setAutoBufferSwap(false);
     }
 
+    void resizeEvent(QResizeEvent *e)
+    {
+        m_thread->lock();
+        QGLWidget::resizeEvent(e);
+        m_thread->unlock();
+    }
+
     void paintEvent(QPaintEvent *)
     {
         m_thread->lock();