patch 9.0.1600: screenpos() does not take w_skipcol into account

Problem:    screenpos() does not take w_skipcol into account.
Solution:   Subtract w_skipcol from column. (closes #12486, closes #12476)

src/version.c

@@ -695,6 +695,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1600,
 /**/
     1599,
 /**/