halign adjust with immutability of style objects in mind

Fixes problems mentioned here: https://github.com/mapnik/mapnik/pull/2426#issuecomment-58771864

okay, no need for action - let's just keep this in mind. If you have a chance would be great to get your feedback over at https://github.com/mapnik/mapnik/pull/2416#issuecomment-58817623.