patch 9.1.0407: Stuck with long line and half-page scrolling

patch 9.1.0407: Stuck with long line and half-page scrolling

Problem:  No scrolling happens with half-page scrolling with line
          filling entire window when 'smoothscroll' is disabled.
          (Mathias Rav, after v9.1.0285)
Solution: Adjust amount to move cursor by so that it is moved the same
          number of lines as was scrolled, even when scrolling different
          number of lines than requested with 'nosmoothscroll'.

fixes: #14743
closes: #14746

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