Handle the case when anchors are no longer trackable

[TrevorFSmith]

In world-scale apps, existing anchors will be become untrackable in some cases like when the user walks out of a room. Right now, the polyfill assumes that anchors are permanently trackable and has no way to indicate otherwise.

Add a flag to anchors aor fire an event when they become untrackable.

[blairmacintyre]

In argon.js, we did two things related to this.

First, the call to determine the rendering coordinates of an anchor (which you should call each frame) would fail if the anchor location relative to rendering coordinates wasn't currently known. This handled both trackable anchors (e.g., image targets) and geospatial anchors when the browser was not giving geospatial tracking data to the app (e.g., when the user told the UA the app couldn't have location access).

We also (for convenience) had "lost" and "found" flags that would be set appropriately the first time an anchor was queried and the state changed, so that the app did not have to keep track of this (otherwise, apps have to implement the obvious pattern to notice state change).