Fixed a bug in the `ARCameraBackground` which would not render the camera texture properly if the `ARSession` had been destroyed and then recreated or if the `ARCameraBackground` had been disabled and then re-enabled.
`ARSubsystemManager.systemState`'s setter was not private, allowing user code to change the system state. The setter is now private.
`ARPlane.trackingState` returned a cached value, which was incorrect if the `ARSession` was no longer active.
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Improvements
Added `ARSession.Reset()` to destroy the current AR Session and establish a new one. This destroys all trackables, such as planes.
Added an `ARSubsystemManager.sessionDestroyed` event. The `ARPlaneManager`, `ARPointCloudManager`, and `ARReferencePointManager` subscribe to this event, and they remove their trackables when the `ARSession` is destroyed.
