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.

5a51fb7777a9950502965a043a70bd6ca5e0498b gnulib-hg