addDecal(decal) {
    const index = UtilsJavascript.arrayCautiousAdd(this.decals, decal);
    if (index === -1) {
      // remove last decal when we've reached the limit
      while (this.decals.length > this.maxNumDecals) {
        this.fadeNextDecal();
      }
    }
  }
Adds a node to the list of decal nodes and auto fades out oldest decal if we've reached the max decals shown on screen.
@param {cc.Node} decal decal node