Fix Scrollback issues.

[tascord]

Fix over-scrollback-ing (sub to saturating_sub), and allow third parties to determine if scrollback should be actionable via screen.scrollback_rows()

[shizhilvren]

Hi Tascord

1. Thanks you try too add a function to get how length we can scrollback. It is usefully.
2. I meet the same reason scrollback issue at versiuon 0.15.2. And after check, author was fix this issue at this commit.

best regards
Shizhilvren

[doy]

yeah, i'm pretty sure this was fixed by #11. thanks though!