code: def deltas(self, start, height):
          
          for y in range(start, min(start + height, self._height)):
              for x in range(self._width):
                  old_cell = self._screen_buffer[y][x]
                  new_cell = self._double_buffer[y][x]
                  if old_cell != new_cell:
                      yield y, x
docs: Return a list-like (i.e. iterable) object of (y, x) tuples