Problem: Cursor position invalid when scrolling with 'smoothscroll' set.
(Ernie Rael)
Solution: Add w_valid_skipcol and clear flags when it changes. Adjust
w_skipcol after moving the cursor.
Problem: With 'smoothscroll' and 'scrolloff' non-zero the cursor position
is not properly adjusted in a long line.
Solution: Move the cursor further up or down in the line.