patch 9.0.1610: display is wrong when 'smoothscroll' is set

Problem:    Display is wrong when 'smoothscroll' is set and scrolling multiple
            lines.
Solution:   Redraw with UPD_NOT_VALID when "skipcol" is or was set.
            (closes #12490, closes #12468)
diff --git a/src/testdir/test_scroll_opt.vim b/src/testdir/test_scroll_opt.vim
index 3806486..f8b4b4f 100644
--- a/src/testdir/test_scroll_opt.vim
+++ b/src/testdir/test_scroll_opt.vim
@@ -798,10 +798,11 @@
 
   let lines =<< trim END
       setlocal cursorline scrolloff=0 smoothscroll
-      call setline(1, repeat([''], 9))
+      call setline(1, repeat([''], 8))
       call setline(3, repeat('a', 50))
-      call setline(8, 'bbb')
-      call setline(9, 'ccc')
+      call setline(4, repeat('a', 50))
+      call setline(7, 'bbb')
+      call setline(8, 'ccc')
       redraw
   END
   call writefile(lines, 'XSmoothMultiSkipcol', 'D')
@@ -811,6 +812,9 @@
   call term_sendkeys(buf, "3\<C-E>")
   call VerifyScreenDump(buf, 'Test_smooth_multi_skipcol_2', {})
 
+  call term_sendkeys(buf, "2\<C-E>")
+  call VerifyScreenDump(buf, 'Test_smooth_multi_skipcol_3', {})
+
   call StopVimInTerminal(buf)
 endfunc