BlackBerry: fix delayed root window posting on OpenGL

The delayed root window posting introduced by
77a06e7e6c broke the OpenGL support. In that patch, the posting of
the root window is trigerred by QQnxRasterRasterBackingStore's call to
QQnxWindow::post(), that obviously does not happen when OpenGL is enabled,
therefore requiring the OpenGL context to explicitly post() the root window.

Task-number: QTBUG-31934

Change-Id: Ifd302c1dde612a03b79c778ec4586aa70f88260d
(cherry picked from commit 6a9333841d)
Reviewed-by: Nicolas Arnaud-Cormos <nicolas@kdab.com>

src/plugins/platforms/qnx/qqnxglcontext.cpp

@@ -315,6 +315,9 @@ void QQnxGLContext::createSurface(QPlatformSurface *surface)
     }
     platformWindow->setBufferSize(surfaceSize);
 
+    // Post root window, in case it hasn't been posted yet, to make it appear.
+    platformWindow->screen()->onWindowPost(platformWindow);
+
     // Obtain the native handle for our window
     screen_window_t handle = platformWindow->nativeHandle();