Improve creating wxGCDC from an existing wxGraphicsContext

Avoid creating a default wxGraphicsContext unnecessarily, only to
immediately delete it and replace it with the provided one.

This was at best unnecessary and at worst resulted in a crash if the
default context couldn't be created, as happened on a headless system
(where wxImage-based wxGraphicsContext can still be created
successfully).

include/wx/dcgraph.h

@@ -60,6 +60,10 @@ public:
 #if defined(__WXMSW__) && wxUSE_ENH_METAFILE
     wxGCDCImpl( wxDC *owner, const wxEnhMetaFileDC& dc );
 #endif
+
+    // Ctor using an existing graphics context given to wxGCDC ctor.
+    wxGCDCImpl(wxDC *owner, wxGraphicsContext* context);
+
     wxGCDCImpl( wxDC *owner );
 
     virtual ~wxGCDCImpl();