patch 8.1.1493: redrawing with popups is slow and causes flicker

Problem:    Redrawing with popups is slow and causes flicker.
Solution:   Avoid clearing and redrawing using a zindex mask.
diff --git a/src/popupmnu.c b/src/popupmnu.c
index 276d49f..09faa69 100644
--- a/src/popupmnu.c
+++ b/src/popupmnu.c
@@ -431,6 +431,12 @@
 						    / (pum_size - pum_height);
     }
 
+#ifdef FEAT_TEXT_PROP
+    // The popup menu is drawn over popup menus with zindex under
+    // POPUPMENU_ZINDEX.
+    screen_zindex = POPUPMENU_ZINDEX;
+#endif
+
     for (i = 0; i < pum_height; ++i)
     {
 	idx = i + pum_first;
@@ -611,6 +617,10 @@
 
 	++row;
     }
+
+#ifdef FEAT_TEXT_PROP
+    screen_zindex = 0;
+#endif
 }
 
 /*