Re: [mediacapture-image] Ignore PTZ request if no PTZ camera (#246)

I fear we may have hit the limits of normative prose in field descriptions, and may need to modify algorithms. So for clarity, here's what I think we need:

* At the start of [getUserMedia](https://w3c.github.io/mediacapture-main/getusermedia.html#dom-mediadevices-getusermedia)'s in parallel steps, add something like:
    1. If the user has no PTZ cameras attached, remove all `pan`, `tilt`, and `zoom` members from `constraints.video` (this'll be reflected in `track.getConstraints()` on success only).
      * _**Note:** This removes a site's ability to use `OverconstrainedError` on `pan`, `tilt`, or `zoom`_ to detect PTZ abilities without risking a prompt if no PTZ devices exist.

    2. (Move the [ptz-specific permission request modification](https://w3c.github.io/mediacapture-image/#pan) steps here (or further down where gUM requests permission).

This order should clarify that:
* PTZ constraints are completely ignored unless the user has at least one PTZ camera attached.
* Sites cannot get `OverconstrainedError` on ptz constraints unless the user has a ptz camera attached.
* Users can't be prompted for PTZ permission ("use & move") unless they have a ptz camera attached.
* PTZ constraints still work in `track.applyConstraints` where they may trigger a PTZ permission prompt.


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Received on Monday, 31 August 2020 20:38:47 UTC