patch 9.0.0245: mechanism to prevent recursive screen updating is incomplete

Problem:    Mechanism to prevent recursive screen updating is incomplete.
Solution:   Add "redraw_not_allowed" and set it in build_stl_str_hl().
            (issue #10952)
diff --git a/src/drawscreen.c b/src/drawscreen.c
index 9e8d9ee..a9b3643 100644
--- a/src/drawscreen.c
+++ b/src/drawscreen.c
@@ -3154,7 +3154,7 @@
     win_T	*wp,
     int		type)
 {
-    if (!exiting && wp->w_redr_type < type)
+    if (!exiting && !redraw_not_allowed && wp->w_redr_type < type)
     {
 	wp->w_redr_type = type;
 	if (type >= UPD_NOT_VALID)
@@ -3186,7 +3186,17 @@
     FOR_ALL_WINDOWS(wp)
 	redraw_win_later(wp, type);
     // This may be needed when switching tabs.
-    if (must_redraw < type)
+    set_must_redraw(type);
+}
+
+/*
+ * Set "must_redraw" to "type" unless it already has a higher value
+ * or it is currently not allowed.
+ */
+    void
+set_must_redraw(int type)
+{
+    if (!redraw_not_allowed && must_redraw < type)
 	must_redraw = type;
 }