The canvas has been tainted by cross-origin data for background-blur.js

### Description

On web stories that utilize background blur, a javascript error occurs saying the canvas has been tainted by cross-origin data.

### Reproduction Steps

Example of a web story that uses background blur on the final slide. (Click all the way thru the story to get the error.)

https://pastebin.com/bsLTLjJt

### Relevant Logs

```shell
Failed to execute 'getImageData' on 'CanvasRenderingContext2D': The canvas has been tainted by cross-origin data.
U @ error-reporting.js:206
(anonymous) @ error-reporting.js:294

background-blur.js:144 Uncaught (in promise) DOMException: Failed to execute 'getImageData' on 'CanvasRenderingContext2D': The canvas has been tainted by cross-origin data.
at Eg (https://cdn.ampproject.org/v0/amp-story-1.0.mjs:5:113775)
at https://cdn.ampproject.org/v0/amp-story-1.0.mjs:5:113101
```

### Browser(s) Affected

Chrome, Safari

### OS(s) Affected

Mac OS 10.15.7

### Device(s) Affected

iMac

### AMP Version Affected

_No response_

Start with background-blur.js and reproduce the failure using the web story at the linked Pastebin URL, clicking through to the final slide. Compare the logged getImageData error with the canvas inputs and verify the fix by confirming the story completes without the cross-origin canvas exception in Chrome and Safari.

Written by the indexing model from the issue text.