patch 8.2.1596: using win_screenpos('.') in tests works but is wrong

Problem:    Using win_screenpos('.') in tests works but is wrong.
Solution:   Use win_screenpos(0).
diff --git a/src/version.c b/src/version.c
index b43e393..4725e61 100644
--- a/src/version.c
+++ b/src/version.c
@@ -755,6 +755,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1596,
+/**/
     1595,
 /**/
     1594,