Should we have a key for visibility of the player of a player being backgrounded?

[PCaponetti]

In conversations about player state the state of being backgrounded and/or not visible came up. Should we have signal for this?

Options discussed so far:

• another state in the player state key: this would mean it is mutually exclusive with other keys and only reported on state-interval mode which misses out on value to request mode where dynamic prioritization of requests can be done
• a separate key to report in any mode: we do get the ability to prioritize requests here, but it will break with the paradigm of request and response mode tying the data to the point in time the request was sent instead of data about the segment being requested. It also leads to incomplete data as transitions to and from backgrounded mode can/will happen in between segment requests.