Look into delaying rendering of screenshots when needed

[andrewserong]

For rendering patterns used in WordPress.com (in the Gutenboarding onboarding flow, and in the page layout picker) we sometimes need to use workarounds to get previews loading quickly enough so that mShots can take an accurate screenshot (e.g. to workaround issues like Automattic/wp-calypso#50919).

It would be good to investigate our options for delaying rendering, or how we can tell mShots when a page has finished really loading / rendering before taking a screenshot.

The objective is to ensure that WordPress.com / Jetpack blocks like the Slideshow block or Map block render correctly without needing as many custom / hard-coded workarounds.