Fix Y-sorted root item having modulation applied twice

[kleonc]

Fixes #95667.

For the y-sorted subtree's root modulation is obtained once again after gathering y-sorted items, just like e.g. transform. In the similar manner it needs to be cancelled out.
(again fixing my own mistake, from #78134 🙃)

[clayjohn]

Should be fine to merge after a rebase

[akien-mga]

@kleonc Would you be able to rebase this soon?

[kleonc]

Rebased, and fixed the change as the previous / modulate would cancel out the whole modulation (including the modulation part propagated from the parent non-y-sorted items). Instead, only the single item's (y-sorted subtree root's) modulation is cancelled out now with / ci->modulate.

But before merging: @clayjohn is it fine as is or should we handle potential division by zero? 🤔

[Repiteo]

Thanks!