Allow using wxRendererNative::DrawGauge() for vertical gauges too

It was unexpected that this method could only be used for horizontal
gauges, so make it work for the vertical ones if wxCONTROL_SPECIAL flag
is specified.

Update MSW and generic implementations and the render sample to show a
vertical gauge as well.

interface/wx/renderer.h

@@ -359,6 +359,8 @@ public:
         bar that is drawn as being filled in, @a max must be strictly positive
         and @a value must be between 0 and @a max.
 
+        @c wxCONTROL_SPECIAL must be set in @a flags for the vertical gauges.
+
         @since 3.1.0
      */
     virtual void DrawGauge(wxWindow* win,