patch 9.1.0215: Half-page scrolling does not support smooth-scrolling

Problem:  Page-wise scrolling with Ctrl-D/Ctrl-U implements
          it's own logic to change the topline and cursor.
          More logic than necessary for scrolling with Ctrl-F/Ctrl-B
          was removed in patch 9.1.0211.
Solution: Re-use the logic from Ctrl-E/Ctrl-Y/Ctrl-F/Ctrl-B while
          staying backward compatible as much as possible.
          Restore some of the logic that determined how many lines will
          be scrolled (Luuk van Baal)

closes: #14316

Signed-off-by: Luuk van Baal <luukvbaal@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/mouse.c b/src/mouse.c
index af14af0..247c6df 100644
--- a/src/mouse.c
+++ b/src/mouse.c
@@ -1129,7 +1129,7 @@
 	if (!(State & MODE_INSERT) && (mouse_vert_step < 0 || shift_or_ctrl))
 	{
 	    // whole page up or down
-	    onepage(cap->arg == MSCR_UP ? FORWARD : BACKWARD, 1L);
+	    pagescroll(cap->arg == MSCR_UP ? FORWARD : BACKWARD, 1L, FALSE);
 	}
 	else
 	{