This requires quite a bit more thorough cleanup. I got it to the point where the titlepic appears after restarting, but it still crashes when starting the game so there's more data that needs to be cleaned up...
This is done to encapsulate the gory details of tag search in one place so that the implementation of multiple tags per sector remains contained to a few isolated spots in the code.
This also moves the special 'tag == 0 -> activate backsector' handling into the iterator class.
