Install mutation observer if document is already ready
We called registerAllPlayers()
unconditionally but forgot
to also register the mutation observer in this case.
Noticed that vimeo embedded player wasn’t controlled by p-b-i.
Reproducible by opening e.g. https://player.vimeo.com/video/346920378 (dont mind the video, just googled vimeo kde lol) in a new tab.