use IsShownOnScreen() in OnSize() to ensure that the window is realized (important for X11-based ports) before trying to create an associated context

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@45488 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

samples/opengl/cube/cube.cpp

@@ -304,7 +304,7 @@ void TestGLCanvas::OnSize(wxSizeEvent& event)
     // don't prevent default processing from taking place
     event.Skip();
 
-    if ( !IsShown() )
+    if ( !IsShownOnScreen() )
         return;
 
     // set GL viewport (not called by wxGLCanvas::OnSize on all platforms...)