patch 9.1.0260: Problems with "zb" and scrolling to new topline with 'smoothscroll'

patch 9.1.0260: Problems with "zb" and scrolling to new topline with 'smoothscroll'

Problem: "zb" does not reveal filler lines at the start of a buffer.
          Scrolled cursor position with 'smoothscroll' is unpredictable,
          and may reset skipcol later if it is not visible (after v9.1.258)
Solution: Replace confusing for loop that reaches final control value too
          early with while loop. Set "w_curswant" accordingly so cursor
          will be placed in visible part of topline.
          (Luuk van Baal)

closes: #14394

Signed-off-by: Luuk van Baal <luukvbaal@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>