amp-img, when used with thumbor, gets an "Malformed URL" error

## What's the issue?

- amp-img, when used with [thumbor](https://github.com/thumbor/thumbor), gets an "Malformed URL" error
- original link works: https://img.vinshine.com/g3PaduaVx6xYAldjNou6Vq7YUMs=/filters:watermark(ngontinh-prod/icon/logo-text.png,-10,-10,10,30)/ngontinh-prod%2Fstory%2Fthumbnail%2Fac-nhan-thanh-doi.jpg
- after deploying and using amp cdn, the link becomes like this and no longer works because some characters have been altered: https://img-vinshine-com.cdn.ampproject.org/i/s/img.vinshine.com/g3PaduaVx6xYAldjNou6Vq7YUMs=/filters:watermark%28ngontinh-prod/icon/logo-text.png,-10,-10,10,30%29/ngontinh-prod/story/thumbnail/ac-nhan-thanh-doi.jpg
- thumbor logs
```
│ 2021-01-04 05:21:18 thumbor:WARNING Malformed URL: /g3PaduaVx6xYAldjNou6Vq7YUMs=/filters:watermark%28ngontinh-prod/icon/logo-text.png,-10,-10,10,30%29/ngontinh-prod/story/thumbnail/ac-nhan-thanh-doi.jpg │
│ 2021-01-04 05:21:18 tornado.access:WARNING 400 GET /g3PaduaVx6xYAldjNou6Vq7YUMs=/filters:watermark%28ngontinh-prod/icon/logo-text.png,-10,-10,10,30%29/ngontinh-prod/story/thumbnail/ac-nhan-thanh-doi.jpg (10.42.0.1) │
│ 2.41ms
```

## How do we reproduce the issue?

- use thumbor and amp-img together

## What browsers are affected?

All browsers

## Which AMP version is affected?

Version 2011252111003

Start by reproducing the issue with amp-img and thumbor using the supplied original and AMP CDN URLs, then trace where the URL is transformed. Done means the thumbor URL remains valid through AMP CDN delivery and no longer produces the reported "Malformed URL" error.

Written by the indexing model from the issue text.