Actor z-index throws an error if set on a child actor or on an Actor not part of a scene

[eonarheim]

This issue was brought up on the forum by Maxime, https://groups.google.com/forum/#!topic/excaliburjs/NPdFFSPDdwE

This may take some work to fix, probably by generalizing draw code from scene and extracting them into a separate class that can be used both by scenes, child actors, and any other actor based data structrue meant for drawing.

Steps to Reproduce

Repro codepen https://codepen.io/anon/pen/YrVwOe?editors=0010#0

Setting the z-index on a child actor does not work because the scene drawTree relies on a scene to order drawing, but child actors do not have a scene they are a member of.

Excalibur Actor.ts line 620

   /**
    * Sets the z-index of an actor and updates it in the drawing list for the scene. 
    * The z-index determines the relative order an actor is drawn in.
    * Actors with a higher z-index are drawn on top of actors with a lower z-index
    * @param newIndex new z-index to assign
    */
   public setZIndex(newIndex: number) {
      this.scene.cleanupDrawTree(this);
      this._zIndex = newIndex;
      this.scene.updateDrawTree(this);
   }

Expected Result

Excalibur should allow z-index operations in child actors

Actual Result

Errors are thrown and the game does not load.

Environment

• browsers and versions: Chrome 60+
• operating system: Windows 10
• Excalibur versions: v0.12.0

[eonarheim]

This also happens if the actor is not part of scene when you set the z-index