Get a length limited unit name for drawing units.

def get_unit_name(self, surf, name, radius):
    """Get a length limited unit name for drawing units."""
    key = (name, radius)
    if key not in self._name_lengths:
      max_len = surf.world_to_surf.fwd_dist(radius * 1.6)
      for i in range(len(name)):
        if self._font_small.size(name[:i + 1])[0] > max_len:
          self._name_lengths[key] = name[:i]
          break
      else:
        self._name_lengths[key] = name
    return self._name_lengths[key]