We use 'wasm-bindgen-futures' as our futures executor on web, which
in turn uses 'queueMicroTask'. This can result in the browser executing
one of our futures while we're still inside our `requestAnimationFrame`
callback (in particular, while we still have the `Player` mutex locked).
We now detect this condition by attempting the lock the Player mutex
inside of our `spawn_local` future. If this fails, we `await`
a `setTimeout`-based promise, which ensures that our code runs in
a new top-level `setTimeout` javascript 'task' (outside of our
`requestAnimationFrame` callback).
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