patch 8.0.0683: visual bell flashes too quickly

Problem:    When using a visual bell there is no delay, causing the flash to
            be very short, possibly unnoticeable.  Also, the flash and the
            beep can lockup the UI when repeated often.
Solution:   Do the delay in Vim or flush the output before the delay. Limit the
            bell to once per half a second. (Ozaki Kiichi, closes #1789)

src/term.c

@@ -2513,6 +2513,75 @@ out_str_nf(char_u *s)
 }
 #endif
 
+/*
+ * A conditional-flushing out_str, mainly for visualbell.
+ * Handles a delay internally, because termlib may not respect the delay or do
+ * it at the wrong time.
+ * Note: Only for terminal strings.
+ */
+    void
+out_str_cf(char_u *s)
+{
+    if (s != NULL && *s)
+    {
+	char_u *p;
+
+#ifdef FEAT_GUI
+	/* Don't use tputs() when GUI is used, ncurses crashes. */
+	if (gui.in_use)
+	{
+	    out_str_nf(s);
+	    return;
+	}
+#endif
+	if (out_pos > OUT_SIZE - 20)
+	    out_flush();
+#ifdef HAVE_TGETENT
+	for (p = s; *s; ++s)
+	{
+	    /* flush just before delay command */
+	    if (*s == '$' && *(s + 1) == '<')
+	    {
+		char_u save_c = *s;
+		int duration = atoi((char *)s + 2);
+
+		*s = NUL;
+		tputs((char *)p, 1, TPUTSFUNCAST out_char_nf);
+		*s = save_c;
+		out_flush();
+#ifdef ELAPSED_FUNC
+		/* Only sleep here if we can limit this happening in
+		 * vim_beep(). */
+		p = vim_strchr(s, '>');
+		if (p == NULL || duration <= 0)
+		{
+		    /* can't parse the time, don't sleep here */
+		    p = s;
+		}
+		else
+		{
+		    ++p;
+		    do_sleep(duration);
+		}
+#else
+		/* Rely on the terminal library to sleep. */
+		p = s;
+#endif
+		break;
+	    }
+	}
+	tputs((char *)p, 1, TPUTSFUNCAST out_char_nf);
+#else
+	while (*s)
+	    out_char_nf(*s++);
+#endif
+
+	/* For testing we write one string at a time. */
+	if (p_wd)
+	    out_flush();
+    }
+}
+
 /*
  * out_str(s): Put a character string a byte at a time into the output buffer.
  * If HAVE_TGETENT is defined use the termcap parser. (jw)