src/debug/traces: use white color instead of black when no worker is found, and set the fillcolor instead of color so as to draw a border

src/debug/traces/starpu_fxt.c

@@ -1629,7 +1629,7 @@ static void handle_tag_done(struct fxt_ev_64 *ev, struct starpu_fxt_options *opt
 	}
 	else
 	{
-		colour= (worker < 0)?"0.0,0.0,0.0":get_worker_color(worker);
+		colour= (worker < 0)?"white":get_worker_color(worker);
 	}
 
 	_starpu_fxt_dag_set_tag_done(tag_id, colour);

src/debug/traces/starpu_fxt_dag.c

@@ -87,14 +87,14 @@ void _starpu_fxt_dag_add_task_deps(unsigned long dep_prev, unsigned long dep_suc
 void _starpu_fxt_dag_set_tag_done(uint64_t tag, const char *color)
 {
 	if (out_file)
-		fprintf(out_file, "\t \"tag_%llx\" [ style=filled, color=\"%s\"]\n",
+		fprintf(out_file, "\t \"tag_%llx\" [ style=filled, fillcolor=\"%s\"]\n",
 			(unsigned long long)tag, color);
 }
 
 void _starpu_fxt_dag_set_task_done(unsigned long job_id, const char *label, const char *color)
 {
 	if (out_file)
-		fprintf(out_file, "\t \"task_%lu\" [ style=filled, label=\"%s\", color=\"%s\"]\n", job_id, label, color);
+		fprintf(out_file, "\t \"task_%lu\" [ style=filled, label=\"%s\", fillcolor=\"%s\"]\n", job_id, label, color);
 }
 
 void _starpu_fxt_dag_add_sync_point(void)