patch 9.1.0280: several issues with 'smoothscroll' support

Problem:  Logic to make sure cursor is in visible part of the screen after
          scrolling the text with 'smoothscroll' is scattered, asymmetric
          and contains bugs.
Solution: Adjust and create helper function for 'smoothscroll' cursor logic.
          (Luuk van Baal)

closes: #14410

Signed-off-by: Luuk van Baal <luukvbaal@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/version.c b/src/version.c
index 6f49e11..5f0c909 100644
--- a/src/version.c
+++ b/src/version.c
@@ -705,6 +705,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    280,
+/**/
     279,
 /**/
     278,