Fix vertical wheel scrolling direction in wxGenericCalendarCtrl

Make scrolling upwards (positive delta) go to the past (i.e. decrease the
date) for consistency with the native calendar controls.

See 50daf1feab (commitcomment-13087042)

src/generic/calctrlg.cpp

@@ -1519,7 +1519,10 @@ void wxGenericCalendarCtrl::OnWheel(wxMouseEvent& event)
     switch ( event.GetWheelAxis() )
     {
         case wxMOUSE_WHEEL_VERTICAL:
-            span = wxDateSpan::Month();
+            // For consistency with the native controls, scrolling upwards
+            // should go to the past, even if the rotation is positive and
+            // could be normally expected to increase the date.
+            span = -wxDateSpan::Month();
             break;
 
         case wxMOUSE_WHEEL_HORIZONTAL: