Properly handle scroll offsets in hit testing

Scroll roots are no longer nested containers holding items, so instead
we need to track the offsets of each, carefully handling fixed position
items and stacking contexts that create new reference frames.
Additionally, we remove the complexity of the pre-computed page scroll
offset, instead opting to send script scrolls to the layout task in
order to more quickly have a ScrollState there that matches the
script's idea of the scroll world.

Fixes #16405.

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