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/ui.c b/src/ui.c
index 896ab0c..00e8887 100644
--- a/src/ui.c
+++ b/src/ui.c
@@ -1498,7 +1498,7 @@
 
 #ifdef FEAT_TEXT_PROP
     // this goes on top of all popup windows
-    screen_zindex = 32000;
+    screen_zindex = CLIP_ZINDEX;
 
     if (col < cbd->min_col)
     {