Fixes to OpenGL samples to avoid asserts/crashes.

Don't call wxGLCanvas::SetCurrent() when the window is not shown.

Closes #13424.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@68909 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

samples/opengl/cube/cube.cpp

@@ -323,10 +323,11 @@ void TestGLCanvas::OnPaint(wxPaintEvent& WXUNUSED(event))
     // is wrong when next another canvas is repainted.
     const wxSize ClientSize = GetClientSize();
 
+    TestGLContext& canvas = wxGetApp().GetContext(this);
     glViewport(0, 0, ClientSize.x, ClientSize.y);
 
     // Render the graphics and swap the buffers.
-    wxGetApp().GetContext(this).DrawRotatedCube(m_xangle, m_yangle);
+    canvas.DrawRotatedCube(m_xangle, m_yangle);
     SwapBuffers();
 }