Fixed zz doesn't center the due to wrapped line

[chomosuke]

When there's wrapped line, sometimes scroll_one_line scroll more than one graphical line, this causes relative_line to be recorded wrong.

This means that if you're dealing with files with a lot of very long lines, typing zz scrolls more than it should. Sometimes no matter how many times you type zz you never get the cursor to center, it simply oscillate as it overshoots every time.