Implement GetPartialTextExtents() in wxGtkPrinterDC

Generic implementation from wxDCImpl doesn't work well with wxPrinterDC
under wxGTK.
(This implementation is adopted from wxCairoContext::GetPartialTextExtents.)

include/wx/gtk/print.h

@@ -291,6 +291,7 @@ protected:
                      wxCoord *descent = NULL,
                      wxCoord *externalLeading = NULL,
                      const wxFont *theFont = NULL ) const wxOVERRIDE;
+    bool DoGetPartialTextExtents(const wxString& text, wxArrayInt& widths) const wxOVERRIDE;
     void DoGetSize(int* width, int* height) const wxOVERRIDE;
     void DoGetSizeMM(int *width, int *height) const wxOVERRIDE;