Re: [mediacapture-image] Clarify meaning of PTZ constraints presence (#256)

> To me, ... {pan: true} means that PTZ is required and, when not available, gUM() fails with OverconstrainedError if used in the basic set (the set is ignored if used in an advanced set) 

I find no support in the spec for that interpretation.

The spec [says](https://w3c.github.io/mediacapture-image/#dom-mediatrackconstraintset-pan): _"A value of `true` is normalized to a value of empty ConstrainDouble"_, which I don't see being able to cause `OverconstrainedError`.

> This would mean that there is no way to require the PTZ capability, which is the intended result of the proposal.

I see not way in the current spec to require PTZ capability without setting a value, e.g. `pan: {exact: 3600}` (it's not clear if `pan: {exact: 0}` does).

Option 3 would add that with e.g. `{pan: {exact: true}}`, modulo whatever we come up with in https://github.com/w3c/mediacapture-image/issues/246.

> Not sure if the extra complexity of allowing pan to be used this way or in the usual DoubleConstraint way is worth it over using a separate constraint for the PTZ capability.

You're listing the natural outcomes of the existing constraints algorithm.

I think the complexity is exactly the same as having a separate boolean constraint, except we avoid some nonsense combos:
```js
{video: {ptz: {exact: false}, pan: {min: 3600}}} 
```

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Received on Wednesday, 9 September 2020 00:00:12 UTC