feat: Shutdown propogation and thread count arg

The Stop command will be handled specially by the Dispatcher to act as a
sort of broadcast into the thread pool. Instead of asking the caller to
feed in the appropriate number of stop commands, a single one will queue
up a stop for all threads. Important for the ergonomics of having a
variable number of threads (instead of the earlier magic constant).
Moreover, it's necessary if the dispatcher stops using the round-robin
style assignment.

The pool size is specifiable as an argument to Dispatcher::new().

src/main.rs

@@ -66,21 +66,16 @@ fn main() {
     };
 
     thread::scope(|s| {
-        let (mut dispatcher, scanline_receiver) = thread_utils::Dispatcher::new(&small_rng);
+        let (mut dispatcher, scanline_receiver) = thread_utils::Dispatcher::new(&small_rng, 4);
 
         s.spawn(move || {
             for y in (0..image.1).rev() {
                 eprintln!("Submitting scanline: {}", y);
                 let job = RenderCommand::Line { line_num: y, context: context.clone() };
-                dispatcher.submit_job(job);
+                dispatcher.submit_job(job).unwrap();
             }
-            //TODO: Dispatcher shutdown mechanism
-            // Just gonna take advantage of the round-robin dispatching to
-            // get a stop command to each thread
-            dispatcher.submit_job(RenderCommand::Stop);
-            dispatcher.submit_job(RenderCommand::Stop);
-            dispatcher.submit_job(RenderCommand::Stop);
-            dispatcher.submit_job(RenderCommand::Stop);
+
+            dispatcher.submit_job(RenderCommand::Stop).unwrap();
             // ... also I happen to know there are 4 threads.
         });