[css-anchor-position-1] Make snapshotted scroll offset per-axis

[xiaochengh]

For #9239

Define whether there's a snapshotted scroll offset on each axis separately, and then revise the concept of snapshotted scroll snapshot and how it's used accordingly.

[xiaochengh]

Wait, this doesn't seem to work for the multiple anchor case.

Say the element uses anchor --a in the x axis and anchor --b in the y axis, and both anchors are in the same scroller. We still want to do scroll adjustments in both axes, but there's only one default anchor.

I think we should refine it into: do adjustment in an axis if there's reference to any anchor element in the same scroller (but not any descendant scroller) as the default anchor in that axis.

[xiaochengh]

Fixed it, PTAL.