[scroll-animations-1] source is updated upon request and in sync with currentTime

[fantasai]

See #8204

[flackr]

These changes look good, but in the original issue you had also assumed that we minted a new scrolltimeline each time the subject changed #8204 (comment) and we decided in the discussion that it was not a new one and had an identity consistent with animation creation. Does this also need to be clarified?

[flackr]

Here's the relevant text from the css-animations spec:
https://drafts.csswg.org/css-animations/#interface-animationevent:~:text=If%20a%20match%20is%20found%2C%20the%20existing%20animation%20is%20updated

Changes to the animation-name update existing animations by iterating over the new list of animations from last to first, and, for each animation, finding the last matching animation in the list of existing animations. If a match is found, the existing animation is updated using the animation properties corresponding to its position in the new list of animations, whilst maintaining its current playback time as described above.

[flackr]

Oh n/m I didn't catch that you committed the identity thing separately. This looks good.