patch 8.1.1580: cannot make part of a popup transparent

Problem:    Cannot make part of a popup transparent.
Solution:   Add the "mask" option.
diff --git a/src/version.c b/src/version.c
index 2b113af..e7ba35c 100644
--- a/src/version.c
+++ b/src/version.c
@@ -778,6 +778,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1580,
+/**/
     1579,
 /**/
     1578,