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/globals.h b/src/globals.h
index ec42984..ddafcc1 100644
--- a/src/globals.h
+++ b/src/globals.h
@@ -74,6 +74,8 @@
 // Array with size Rows x Columns containing zindex of popups.
 EXTERN short	*popup_mask INIT(= NULL);
 EXTERN short	*popup_mask_next INIT(= NULL);
+// Array with flags for tansparent cells of current popup.
+EXTERN char	*popup_transparent INIT(= NULL);
 
 // Flag set to TRUE when popup_mask needs to be updated.
 EXTERN int	popup_mask_refresh INIT(= TRUE);
@@ -1681,5 +1683,7 @@
 
 #ifdef FEAT_TEXT_PROP
 EXTERN int text_prop_frozen INIT(= 0);
+
+// Set to TRUE if there is any visible popup.
 EXTERN int popup_visible INIT(= FALSE);
 #endif