patch 9.1.1388: Scrolling one line too far with 'nosmoothscroll' page scrolling

Problem:  One-off error in "count" to make "w_skipcol" zero with
          'nosmoothscroll' page scrolling when last virtual line
          in a buffer line is exactly the entire window width.
          (Hirohito Higashi)
Solution: Properly compute the smallest integer value necessary
          to make "w_skipcol" zero (Luuk van Baal)

fixes: #17317
closes: #17318

Signed-off-by: Luuk van Baal <luukvbaal@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/move.c b/src/move.c
index 1c1edde..e3771a5 100644
--- a/src/move.c
+++ b/src/move.c
@@ -3215,10 +3215,10 @@
 
 	int width1 = curwin->w_width - curwin_col_off();
 	int width2 = width1 + curwin_col_off2();
-	count = 1 + (curwin->w_skipcol - width1) / width2;
+	count = 1 + (curwin->w_skipcol - width1 - 1) / width2;
 	if (fixdir == FORWARD)
-	    count = 2 + (linetabsize_eol(curwin, curwin->w_topline)
-					- curwin->w_skipcol - width1) / width2;
+	    count = 1 + (linetabsize_eol(curwin, curwin->w_topline)
+			    - curwin->w_skipcol - width1 + width2 - 1) / width2;
 	scroll_redraw(fixdir == FORWARD, count);
 	*curscount += count * (fixdir == dir ? 1 : -1);
     }