SPT: Make cover block images fill entire block in SPT preview

[frontdevde]

Changes proposed in this Pull Request

This PR aims to provide a pragmatic solution to the issue reported in #39351. With this PR in place, fixed background images on the cover block should fill the entire block in the SPT preview.

For further details please see this in-depth comment.

Testing instructions

• Check out this PR locally and run npx lerna run dev --scope='@automattic/full-site-editing' --stream from Calypso root
• Navigate to Pages > Add New to create a new page
• In the page template selector, scroll down to Home Pages
• Select the Rockfield page template (the one with cook's hands on it)
• Verify that the result you see matches the After state below

Before:

After:

Fixes #39351

[matticbot]

Test live: https://calypso.live/?branch=fix/spt-cover-block-fixed-background

[obenland]

While I couldn't reproduce the original bug with my main machine, I was able to reproduce it with my other one. It's a bummer we can't have both, a full image and a working parallax effect.

I'm approving the technical fix here. Let's wait until @ianstewart had a chance to weigh in at #39351 (comment) before merging it, please.

[frontdevde]

Let's wait until @ianstewart had a chance to weigh in

I've just had the chance to briefly chat to Ian. He was just about to board a transatlantic flight and said that @alaczek is filling in for him while he's away. As she already gave her OK in this comment, I'll go ahead and merge this (especially given that it's a clear improvement over the current state as well).

[matticbot]

This PR does not affect the size of JS and CSS bundles shipped to the user's browser.

Generated by performance advisor bot at iscalypsofastyet.com.