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/window.c b/src/window.c
index 7c5c7ff..9c4a199 100644
--- a/src/window.c
+++ b/src/window.c
@@ -2806,6 +2806,7 @@
 
     if (wp->w_last_topline != wp->w_topline
 	    || wp->w_last_leftcol != wp->w_leftcol
+	    || wp->w_last_skipcol != wp->w_skipcol
 	    || wp->w_last_width != wp->w_width
 	    || wp->w_last_height != wp->w_height)
     {
@@ -2820,6 +2821,7 @@
 	{
 	    wp->w_last_topline = wp->w_topline;
 	    wp->w_last_leftcol = wp->w_leftcol;
+	    wp->w_last_skipcol = wp->w_skipcol;
 	    wp->w_last_width = wp->w_width;
 	    wp->w_last_height = wp->w_height;
 	}