[mediacapture-record] Feature request: Concat filter (#166)

guest271314 has just created a new issue for https://github.com/w3c/mediacapture-record:

== Feature request: Concat filter  ==
Related: https://github.com/w3c/mediacapture-record/issues/147; https://github.com/w3c/mediacapture-fromelement

Feature request: Include an option to `MediaRecorder` to concatenate (e.g., http://trac.ffmpeg.org/wiki/Concatenate) all input streams to a single `webm` file.

For example 

`Promise.all([Promise0, Promise1, Promise2])`

when all `PromiseN` are fulfilled `Promise.all()` is fulfilled, even if `Promise2` is resolved before `Promise0`. 

Such code can be implemented in `MediaRecorder` as

```
let recorder = new MediaRecorder(new MediaStream([video0.captureStream(), video1.captureStream(), video2.captureStream()]), {concat:true, width:<"scaleToMinimumDetected|scaleToMaximumDetected|default:maximum">, height:<"scaleToMinimumDetected|scaleToMaximumDetected|default:maximum">});

recorder.ondataavailable = e => {
 // e.data single `.webm` file
}
```

Please view or discuss this issue at https://github.com/w3c/mediacapture-record/issues/166 using your GitHub account