yield a thread loop or tick in event_broadcastandwait

- yield a thread loop or tick in looks_switchbackdroptoandwait

If all threads broadcastAndWait is watching are "waiting" for a future
frame, yield until the next tick so event_broadcastandwait also defers
to a future frame.

src/blocks/scratch3_event.js

@@ -100,10 +100,20 @@ class Scratch3EventBlocks {
             const instance = this;
             const waiting = util.stackFrame.startedThreads.some(thread => instance.runtime.isActiveThread(thread));
             if (waiting) {
+                // If all threads are waiting for the next tick or later yield
+                // for a tick as well. Otherwise yield until the next loop of
+                // the threads.
+                if (
+                    util.stackFrame.startedThreads
+                        .every(thread => instance.runtime.isWaitingThread(thread))
+                ) {
+                    util.yieldTick();
+                } else {
                     util.yield();
                 }
             }
         }
     }
+}
 
 module.exports = Scratch3EventBlocks;

src/blocks/scratch3_looks.js

@@ -409,9 +409,19 @@ class Scratch3LooksBlocks {
         const instance = this;
         const waiting = util.stackFrame.startedThreads.some(thread => instance.runtime.isActiveThread(thread));
         if (waiting) {
+            // If all threads are waiting for the next tick or later yield
+            // for a tick as well. Otherwise yield until the next loop of
+            // the threads.
+            if (
+                util.stackFrame.startedThreads
+                    .every(thread => instance.runtime.isWaitingThread(thread))
+            ) {
+                util.yieldTick();
+            } else {
                 util.yield();
             }
         }
+    }
 
     nextBackdrop () {
         const stage = this.runtime.getTargetForStage();