Fix uninitialized Y-sort modulate for CanvasItems

[kleonc]

Fix #78035.

All MRPs I've tested in #77079 had top most Y-sorted item without actual visuals so haven't noticed that such root CanvasItem's ysort_modulate remains not initialized to Color(1, 1, 1, 1). And I must have not tested the simplest case of a single Y-sorted node. 🤦‍♂️

Marking as cherrypick:4.0 as #77079 is still labelled as such.

[kleonc]

@akien-mga Actually, initializing ysort_modulate to Color(1, 1, 1, 1) was not a proper fix. It can remain uninitialized for not y-sorted items, that's not a problem. It only needs to additionally be set for the y-sorted branch-root CanvasItem and it needs to be done each time y-sorting is being resolved. When only initializing such branch-root could end up having modulation different than Color(1, 1, 1, 1) after some y-sorting toggling and modulation changes. Rebased with should-be-a-proper-fix.

[akien-mga]

Thanks!

[YuriSizov]

Cherry-picked for 4.0.4 together with #77079.