Improve autoplay for videos outside initial viewport

[swissspidy]

This is a follow-up to #1498 / #1183

autoplay takes precedence over preload. When we know that a video is not in the initial viewport, we could remove autoplay in favor of an IntersectionObserver-based implementation that only autoplays the video when actually visible.

Related to #1035 which involves IntersectionObserver to lazy-load background images.

[westonruter]

Also related, we can lazy-load the poster image when video enters the viewport: #1591.

[westonruter]

Conversely, as I've noted now in the description of #1592:

When a video is in all initial viewports, it should probably get preload=auto especially when it is the LCP element. It may even make sense to add fetchpriority=high preload links when it is the LCP element, particularly when the video has autoplay.