Make ComputeScaleAndOrigin() as virtual part of wxDCBase and this way present on missing platforms. Add into manaul too.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@31034 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/gtk/dc.h

@@ -51,7 +51,7 @@ public:
     // Resolution in pixels per logical inch
     virtual wxSize GetPPI() const;
 
-    virtual bool StartDoc( const wxString& WXUNUSED(message) ) { return TRUE; }
+    virtual bool StartDoc( const wxString& WXUNUSED(message) ) { return true; }
     virtual void EndDoc() { }
     virtual void StartPage() { }
     virtual void EndPage() { }