Improve cleanup on clone disposal

The `dispose()` method on `RenderedTarget` now:
- informs the runtime that it should end any threads corresponding the
  target being disposed, and
- removes the clone from its sprite.

src/sprites/rendered-target.js

@@ -832,6 +832,8 @@ class RenderedTarget extends Target {
      */
     dispose () {
         this.runtime.changeCloneCounter(-1);
+        this.runtime.stopForTarget(this);
+        this.sprite.removeClone(this);
         if (this.renderer && this.drawableID !== null) {
             this.renderer.destroyDrawable(this.drawableID);
             if (this.visible) {

src/sprites/sprite.js

@@ -58,6 +58,18 @@ class Sprite {
         }
         return newClone;
     }
+
+    /**
+     * Disconnect a clone from this sprite. The clone is unmodified.
+     * In particular, the clone's dispose() method is not called.
+     * @param {!RenderedTarget} clone - the clone to be removed.
+     */
+    removeClone (clone) {
+        const cloneIndex = this.clones.indexOf(clone);
+        if (cloneIndex >= 0) {
+            this.clones.splice(cloneIndex, 1);
+        }
+    }
 }
 
 module.exports = Sprite;