patch 9.0.0304: WinScrolled is not triggered when only skipcol changes

Problem:    WinScrolled is not triggered when only skipcol changes.
Solution:   Add w_last_skipcol and use it. (closes #10998)
diff --git a/src/autocmd.c b/src/autocmd.c
index cc9d99d..2e30262 100644
--- a/src/autocmd.c
+++ b/src/autocmd.c
@@ -1249,6 +1249,7 @@
 		{
 		    curwin->w_last_topline = curwin->w_topline;
 		    curwin->w_last_leftcol = curwin->w_leftcol;
+		    curwin->w_last_skipcol = curwin->w_skipcol;
 		    curwin->w_last_width = curwin->w_width;
 		    curwin->w_last_height = curwin->w_height;
 		}