Look into delaying rendering of screenshots when needed

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 https://github.com/Automattic/wp-calypso/issues/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.

Start by reviewing the linked Calypso issue and reproducing the delayed-rendering problem with the Gutenboarding and page layout picker previews. Investigate how mShots can detect that a page has finished loading and rendering, including the Slideshow and Map blocks. Done means a documented, reliable approach that reduces custom or hard-coded workarounds.

Written by the indexing model from the issue text.