Fix scrollback offset bugs

[rezigned]

When scrollback length is larger than visible rows. There're 2 issues that I've found.

Example config

let mut p = vt100::Parser::new(3, n, 10)
p.screen_mut().set_scrollback(6);

1. Extra rows returned
The .skip method will skip the first 4 (10 - 6 = 4) rows, thus it'll return 6 rows in total.
Even though we only want 3.

This is fixed by setting the upper bound via .take(rows_len).

2. Subtraction overflow
The .chain method will take additional rows via rows_len - offset i.e. 3 - 6 = overflow! We just need 0 rows.

This is fixed by using saturation.

Fix #4 #5