Select units within a rectangle.

def select_rect(action, action_space, select_add, screen, screen2):
  """Select units within a rectangle."""
  select = spatial(action, action_space).unit_selection_rect
  out_rect = select.selection_screen_coord.add()
  screen_rect = point.Rect(screen, screen2)
  screen_rect.tl.assign_to(out_rect.p0)
  screen_rect.br.assign_to(out_rect.p1)
  select.selection_add = bool(select_add)