Add wxDC::GetFontMetrics() and implement it for wxMSW.

Add a new wxDC method allowing to retrieve the font characteristics not
available from GetTextExtent(), notably the internal leading (and also the
average font width).

Currently this is implemented for wxMSW only, the internal leading is always 0
in the other ports.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@67063 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/msw/dc.h

@@ -165,6 +165,12 @@ protected:
     void RealizeScaleAndOrigin();
 
 public:
+    virtual void DoGetFontMetrics(int *height,
+                                  int *ascent,
+                                  int *descent,
+                                  int *internalLeading,
+                                  int *externalLeading,
+                                  int *averageWidth) const;
     virtual void DoGetTextExtent(const wxString& string,
                                  wxCoord *x, wxCoord *y,
                                  wxCoord *descent = NULL,