Avoid including X11 headers from <wx/glcanvas.h>

It's generally wxWidgets policy not to include platform-specific headers from our own
to avoid namespace pollution issues, in this case with names like "None" and "Window".

samples/opengl/isosurf/isosurf.cpp

@@ -118,11 +118,7 @@ MyFrame::MyFrame(wxFrame *frame, const wxString& title, const wxPoint& pos,
         { WX_GL_RGBA, WX_GL_MIN_RED, 1, WX_GL_MIN_GREEN, 1,
         WX_GL_MIN_BLUE, 1, WX_GL_DEPTH_SIZE, 1,
         WX_GL_DOUBLEBUFFER,
-#  if defined(__WXMAC__)  || defined(__WXQT__)
-        GL_NONE };
-#  else
-        None };
-#  endif
+        0 };
 #endif
 
     if (!g_doubleBuffer)
@@ -130,7 +126,7 @@ MyFrame::MyFrame(wxFrame *frame, const wxString& title, const wxPoint& pos,
         wxLogWarning("Disabling double buffering");
 
 #ifdef __WXGTK__
-        gl_attrib[9] = None;
+        gl_attrib[9] = 0;
 #endif
         g_doubleBuffer = GL_FALSE;
     }