Better glows (#152)

* Strip out old script glowing in thread management

* Add new tracking mechanism for glowing scripts

* Track parents and use them to determine script glows

* Use top-block for a thread if there's nothing on the stack

* Remove `console.log`

src/engine/thread.js

@@ -28,6 +28,12 @@ function Thread (firstBlock) {
      * @type {number}
      */
     this.status = 0; /* Thread.STATUS_RUNNING */
+
+    /**
+     * Whether the thread requests its script to glow during this frame.
+     * @type {boolean}
+     */
+    this.requestScriptGlowInFrame = false;
 }
 
 /**