patch 8.1.1620: no test for popup window with border and mask

Problem:    No test for popup window with border and mask.
Solution:   Add this popup window, fix problems.
diff --git a/src/version.c b/src/version.c
index e86b5bf..9af8988 100644
--- a/src/version.c
+++ b/src/version.c
@@ -778,6 +778,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1620,
+/**/
     1619,
 /**/
     1618,