Implement DrawSpline in wxSVGFileDC

Drawing Bézier curves is supported natively in SVG so these curves
should be used instead of generic wxDC implementation based on
polygonal approximation.

include/wx/dcsvg.h

@@ -198,6 +198,9 @@ private:
 
     virtual void DoDrawLines(int n, const wxPoint points[],
                              wxCoord xoffset, wxCoord yoffset) wxOVERRIDE;
+#if wxUSE_SPLINES
+    void DoDrawSpline(const wxPointList* points) wxOVERRIDE;
+#endif // wxUSE_SPLINES
 
     virtual void DoDrawPoint(wxCoord x, wxCoord y) wxOVERRIDE;