patch 8.2.4777: screendump tests fail because of a redraw

Problem:    Screendump tests fail because of a redraw.
Solution:   Do not output t_8u before receiving termresponse.  Redraw only
            when t_8u is not reset and termresponse is received.

src/term.c

@@ -165,6 +165,14 @@ static termrequest_T *all_termrequests[] = {
     &winpos_status,
     NULL
 };
+
+// The t_8u code may default to a value but get reset when the term response is
+// received.  To avoid redrawing too often, only redraw when t_8u is not reset
+// and it was supposed to be written.
+// FALSE -> don't output t_8u yet
+// MAYBE -> tried outputing t_8u while FALSE
+// OK    -> can write t_8u
+int write_t_8u_state = FALSE;
 # endif
 
 /*
@@ -2104,6 +2112,7 @@ set_termname(char_u *term)
 #ifdef FEAT_TERMRESPONSE
     LOG_TR(("setting crv_status to STATUS_GET"));
     crv_status.tr_progress = STATUS_GET;	// Get terminal version later
+    write_t_8u_state = FALSE;
 #endif
 
     /*
@@ -3049,6 +3058,8 @@ term_rgb_color(char_u *s, guicolor_T rgb)
 #define MAX_COLOR_STR_LEN 100
     char	buf[MAX_COLOR_STR_LEN];
 
+    if (*s == NUL)
+	return;
     vim_snprintf(buf, MAX_COLOR_STR_LEN,
 				  (char *)s, RED(rgb), GREEN(rgb), BLUE(rgb));
 #ifdef FEAT_VTP
@@ -3078,7 +3089,12 @@ term_bg_rgb_color(guicolor_T rgb)
     void
 term_ul_rgb_color(guicolor_T rgb)
 {
-    term_rgb_color(T_8U, rgb);
+# ifdef FEAT_TERMRESPONSE
+    if (write_t_8u_state != OK)
+	write_t_8u_state = MAYBE;
+    else
+# endif
+	term_rgb_color(T_8U, rgb);
 }
 #endif
 
@@ -4814,8 +4830,11 @@ handle_version_response(int first, int *arg, int argc, char_u *tp)
 	{
 	    set_string_option_direct((char_u *)"t_8u", -1, (char_u *)"",
 								  OPT_FREE, 0);
-	    redraw_later(CLEAR);
 	}
+	if (*T_8U != NUL && write_t_8u_state == MAYBE)
+	    // Did skip writing t_8u, a complete redraw is needed.
+	    redraw_later_clear();
+	write_t_8u_state = OK;  // can otuput t_8u now
 
 	// Only set 'ttymouse' automatically if it was not set
 	// by the user already.

src/version.c

@@ -746,6 +746,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    4777,
 /**/
     4776,
 /**/