Lazy video poster never loads when no media fetch is parked #13083

The poster frame steps park the poster fetch without starting to observe the element:

6. If the will lazy load element steps given the video return true:
   1. Let posterResumptionSteps be the rest of this algorithm starting with the step labeled Fetch.
   2. Set the video element's poster lazy load resumption steps to posterResumptionSteps.
   3. Return.

The only caller of start intersection-observing a lazy loading element for media is the resource fetch algorithm, so the poster only loads if a media resource fetch happens to be parked at the same time. It isn't in these cases:

• <video loading=lazy poster=p.jpg> with no src and no source children: resource selection returns before the resource fetch algorithm, so the poster never loads.
• <video loading=lazy poster=a.jpg src=v.mp4>, scrolled into view and then poster set to b.jpg: the lazy load intersection observer callback already stopped observing the element, so b.jpg never loads.

Maybe step 6 should also start intersection-observing the element? That's a no-op if it's already observed.

cc @scottjehl