[w3c/manifest] Use the newly defined ImageResource (#854)

The `ImageResource` definition moved to its own [spec](https://w3c.github.io/image-resource/). The Manifest's definition includes some extra manifest-specific fields. So instead of defining its own `ImageResource` it could re-use parts of the new `ImageResource` by defining the following dictionary:

```webidl
dictionary ManifestImageResource : ImageResource {
  USVString platform;
  USVString purpose;
}
```

-- 
You are receiving this because you are subscribed to this thread.
Reply to this email directly or view it on GitHub:
https://github.com/w3c/manifest/issues/854