avoid creating and immediately destroying a wxGraphicsContext when creating a wxDC with wxGTK3

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72073 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/dcgraph.h

@@ -201,6 +201,10 @@ public:
 #endif // __WXMSW__
 
 protected:
+    // unused int parameter distinguishes this version, which does not create a
+    // wxGraphicsContext, in the expectation that the derived class will do it
+    wxGCDCImpl(wxDC* owner, int);
+
     // scaling variables
     bool m_logicalFunctionSupported;
     wxGraphicsMatrix m_matrixOriginal;