send exit messages to threads

src/main.rs

@@ -52,8 +52,14 @@ fn main() -> Result<(), std::io::Error> {
     for (x, y, p) in image.enumerate_pixels() {
         let _ = sender.send(Message::Pixel(x, y, p.clone()));
     }
+
+    (0..16).for_each(|_| {
+        let _ = sender.send(Message::Exit);
+    });
+
     for thread in handles {
         let _ = thread.join();
     }
+
     Ok(())
 }