Add simple FreeType font cache in class ft_manager. * libinterp/corefcn/txt-eng-ft.cc (ft_face_destroyed): New static function. (ft_manager::font_destroyed): New static method. (ft_manager::do_font_destroyed): New method. (ft_manager::ft_key, ft_manager::ft_cache): New typedef's. (ft_manager::cache): New member, storing weak references to loaded fonts. (ft_manager::do_get_font): Look for font into the cache. Use fontconfig and freetype if not found. Insert newly loaded fonts into the cache. Install finalizer to update the cache on font destruction.