Re: [webrtc-pc] Deciding on resolutions with scaleResolutionDownBy

What about the third case?
> * The encoder doesn't handle scaling to certain resolutions 
>   * webrtc.org requires that all layers have identical aspect ratio, see [1]

Rounding to the nearest integer can create unintended aspect changes. Consider:

```js
await sender.setParameters({encodings: [
  { rid: "high" },                             // 320x240
  { rid: "medium", scaleResolutionDownBy: 2 }, // 160x120 (320*120 - 160*240 = 0)
  { rid: "low", scaleResolutionDownBy: 3 },    // 107x80  (320*80 - 107*240 = -80)
]});
```

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