[mythtv] Button backgrounds not working

[Tom van Wietmarschen]

Does anyone else have a problem with the backgrounds for buttons ?
They just don't show and I can't see why, I've tried looking for clues in 
themedmenu.cpp but I see no reason why it shouldn't work. 
also, when I rename button_off.png there's no complaint about a missing image
whereas it does complain when I remove button_on.png, so apparently there isn't
even an attempt to load it.

Tom