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/vim.h b/src/vim.h
index 481d065..cb1d389 100644
--- a/src/vim.h
+++ b/src/vim.h
@@ -2602,4 +2602,6 @@
 #define APC_SAVE_FOR_UNDO	1   // call u_savesub() before making changes
 #define APC_SUBSTITUTE		2   // text is replaced, not inserted
 
+#define CLIP_ZINDEX 32000
+
 #endif /* VIM__H */