Implement shadow buffer to stora changes in memory until update() method is called

main.cpp

@@ -120,6 +120,8 @@ int main(int argc, char* argv[]) {
                       tm->tm_hour, tm->tm_min, tm->tm_sec);
         draw_centered(display, 41, line_buf);
 
+        display.update();
+
         // --- Sleep aligned to next whole second ---
         struct timespec ts{};
         clock_gettime(CLOCK_REALTIME, &ts);