fix: respect the unidirectional nature of time

[Rich-Harris]

supersedes #4531, fixes #4468. It turns out that performance.now() uses a different clock to requestAnimationFrame, or at least they behave as though that were true. You can see if yourself by doing this:

function go() {
  const then = performance.now();

  requestAnimationFrame((now) => {
    if (now < then) {
      console.error('oops! rift detected in spacetime continuum');
    }
  });
}

If you call go() enough times, you'll likely see the error, especially in Chrome.

#4531 offers one fix, but I think this one is probably more future-proof, and means that the first frame of a spring loop 'does' something which likely isn't true with the alternative.

No test because I have no idea how.

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[Rich-Harris]

preview: https://svelte-dev-git-preview-svelte-14541-svelte.vercel.app/

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