[img-conversion] toBlob() Having two optional kinda sucks... let's merge them. (#13)

right now: 

```
  Promise<Blob> toBlob (optional DOMString type, optional EncoderOpts options);
```

Why not just add the `type` member to `EncoderOpts`? It will work the same as currently specified, as when missing, just assume type="image/png".
   


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Reply to this email directly or view it on GitHub:
https://github.com/WICG/img-conversion/issues/13