make x11 dc could get cairo context, so the graphic module could use in x11. This use cairo xlib backend.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@77853 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/x11/dcclient.h

@@ -108,6 +108,8 @@ public:
 
     virtual void ComputeScaleAndOrigin();
 
+    virtual void* GetCairoContext() const wxOVERRIDE;
+
 protected:
     // implementation
     // --------------