Autopause hides custom poster image

[jannispaul]

When the autopause attribute is applied to the component, the custom poster image flashes briefly but then disappears without any user interaction. It took me some time to figure out the cause, so I believe it would be helpful to at least document this behavior.

Thanks for building this awesome component—it's greatly appreciated!

[justinribeiro]

Auto-pause relies on the Intersection Observer, which is why the custom poster hides (because the iframe loads and then binds to the postMessage for pause controls. Will update docs for this!