patch 8.2.5128: syntax disabled when using synID() in searchpair() skip expr

Problem:    Syntax highlighting disabled when using synID() in searchpair()
            skip expression and it times out. (Jaehwang Jung)
Solution:   Add the redrawtime_limit_set flag. (closes #10562)
diff --git a/src/drawscreen.c b/src/drawscreen.c
index 2c184bd..4fea562 100644
--- a/src/drawscreen.c
+++ b/src/drawscreen.c
@@ -2179,6 +2179,7 @@
 #endif
 #ifdef SYN_TIME_LIMIT
     // Set the time limit to 'redrawtime'.
+    redrawtime_limit_set = TRUE;
     init_regexp_timeout(p_rdt);
 #endif
 #ifdef FEAT_FOLDING
@@ -2692,6 +2693,7 @@
 
 #ifdef SYN_TIME_LIMIT
     disable_regexp_timeout();
+    redrawtime_limit_set = FALSE;
 #endif
 
     // Reset the type of redrawing required, the window has been updated.
diff --git a/src/globals.h b/src/globals.h
index 63377af..888f6e9 100644
--- a/src/globals.h
+++ b/src/globals.h
@@ -1228,6 +1228,10 @@
 #ifdef FEAT_DIFF
 EXTERN int	need_diff_redraw INIT(= 0); // need to call diff_redraw()
 #endif
+#ifdef FEAT_RELTIME
+// flag set when 'redrawtime' timeout has been set
+EXTERN int	redrawtime_limit_set INIT(= FALSE);
+#endif
 
 EXTERN int	need_highlight_changed INIT(= TRUE);
 
diff --git a/src/syntax.c b/src/syntax.c
index 679b99d..c2e83e6 100644
--- a/src/syntax.c
+++ b/src/syntax.c
@@ -3150,8 +3150,8 @@
     colnr_T	col,
     syn_time_T  *st UNUSED)
 {
-    int r;
-    int timed_out = FALSE;
+    int		r;
+    int		timed_out = FALSE;
 #ifdef FEAT_PROFILE
     proftime_T	pt;
 
@@ -3181,7 +3181,7 @@
     }
 #endif
 #ifdef FEAT_RELTIME
-    if (timed_out && !syn_win->w_s->b_syn_slow)
+    if (timed_out && redrawtime_limit_set && !syn_win->w_s->b_syn_slow)
     {
 	syn_win->w_s->b_syn_slow = TRUE;
 	msg(_("'redrawtime' exceeded, syntax highlighting disabled"));
diff --git a/src/version.c b/src/version.c
index 39bbd4d..75cbbaf 100644
--- a/src/version.c
+++ b/src/version.c
@@ -735,6 +735,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    5128,
+/**/
     5127,
 /**/
     5126,