fix: don't cancel transition of already outroing elements

[dummdidumm]

#16977 forgot one detail: While an element is outroing, the block of e.g. an if block can be triggered again, resolving to the same condition. In that case we have an in-between state where the element is still onscreen but already outroing. We have to detect this to not eagerly destroy the corresponding effect when we arrive at the outro/destroy logic.

Fixes #16982

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