[Bug]: Video atoms poster frame dependent on viewport

### What happened?

Steps to repro:
1. Open any page with proper media atom
2. Grab browser window, open DevTools or otherwise change the viewport size
3. Observe poster frame resizing


### Where is this happening?

https://www.theguardian.com/video (cards with active video)
https://www.theguardian.com/politics/video/2023/oct/04/young-conservatives-the-brexit-generation-at-tory-party-conference-video
or any other page with video atoms (following [this](https://github.com/guardian/dotcom-rendering/pull/8969))

### What is the impact?

Minimal.