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)
diff --git a/src/version.c b/src/version.c
index 872587e..b854893 100644
--- a/src/version.c
+++ b/src/version.c
@@ -696,6 +696,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1600,
+/**/
     1599,
 /**/
     1598,