TextIter.backwardVisibleLines

Moves count visible lines backward, if possible.

If count would move past the start or end of the buffer, moves to the start or end of the buffer.

The return value indicates whether the iterator moved onto a dereferenceable position; if the iterator didn’t move, or moved onto the end iterator, then false is returned. If count is 0, the function does nothing and returns false. If count is negative, moves forward by 0 - count lines.