Call-back when the AR system of the device (ARCore / ARKit) sends the anchor ID to our scene, so that we can link it to the entity.

AnchorPositioning.js

// Anchor registration callback. Sets the anchor ID of the entity's
// ArAnchorComponent. The engine's ArSystem will automatically update
// the world position and orientation of entities with a valid anchor ID.
registerAnchorCallback(anchorId) {
	if (anchorId) {
		this.internalAnchorEntity.getComponent('ArAnchorComponent').anchorId = anchorId;
		console.log("Registered AR anchor: " + anchorId);
		this.placeMode.set(false);
		this.worldPlacedEvent.emit();
		console.log("Quit place mode and sent WorldPlaced event");
	} else {
		console.log("No Anchor ID received");
	}
}