code: public Layer setDepth(float depth) {
      float oldDepth = this.depth;
      if (depth != oldDepth) {
        this.depth = depth;
        if (parent != null) parent.depthChanged(this, oldDepth);
      }
      return this;
    }
docs: Sets the depth of this layer.
  <p>
  Within a single {@link GroupLayer}, layers are rendered from lowest depth to highest depth.
  
  @return a reference to this layer for call chaining.