xb-anchors: spatial anchors & persistence

An anchor tells the platform "keep this exactly here". As the headset improves its understanding of the room, anchored content is corrected along with it, which plain world coordinates are not. With persistence enabled, anchors can also be restored in a later session, so an app can remember where things were left. See demos/anchors.

Why not just store coordinates

World coordinates are relative to the session's tracking origin. That origin is not guaranteed to be the same next time, so coordinates saved to storage can come back meaning a different place. Anchors avoid this: the platform re-localises them.

Enable