Animation rects need to be measured before any other updates

[Rich-Harris]

Describe the bug
When using the animate: directive, DOM nodes are measured immediately before updates are applied to them. But if an update to a parent element already happened, this can result in them being in the wrong place at the time of being measured.

To Reproduce
https://svelte.dev/repl/8c4d61bba033437bb9add90f5355df25?version=3.21.0 (via https://stackoverflow.com/questions/61430023/svelte-out-transition-in-flip-animation-in-grid-layout-flies-off-from-top-left)

Expected behavior
Toggling the column should cause 3, 6 and 9 to smoothly move to their new location. To achieve this, I think we'd need to measure DOM nodes before any DOM updates happened at all, which sounds tricky to do.

Severity
Medium. It's a bad bug, but a fairly niche situation.

[Conduitry]

This seems sort of adjacent to #4111/#4344 but it might not actually be related.