Move feathers propagation into PostUpdate

[alice-i-cecile]

Objective

• As discussed in Make HierarchyPropagatePlugin scheduling configurable #20433 (comment), these probably belong in PostUpdate to ensure that spawned objects in Update have the correct values immediately without the need for ordering.
• Finishes and closes Generic propagation should happen in PostUpdate #20356.

Solution

• Change the schedule from Update to PostUpdate

Testing

• I'm not sure exactly how to test this effectively: @viridia, can you advise?

[ickshonpe]

The propagation systems for TextFont need to run in UiSystems::Propagate so the fonts are up-to-date for measure_text_system and detect_text_needs_rerender in UiSystems::Content.

TextColor it doesn't matter though, the colors are only needed during extraction.

[viridia]

Yeah, what @ickshonpe said.

Testing: is pretty easy if you look fast. Run the feathers example without this fix - right at the start, you'll see all of the text in a different font, and then one frame later (one long frame - the app is still starting up so things are a bit slow) the text will "pop" into the correct font. Apply the fix, and this problem goes away.

The problem is even more evident in the popup menu example - but that only runs in cart's BSN branch.

[alice-i-cecile]

Awesome, thank you both. I'll get to that tomorrow.

[alice-i-cecile]

I've applied the fix, and cannot reproduce the bug on this branch. I also couldn't reproduce it on main, but this is definitely a better plan regardless so I'm not too upset about that.

[viridia]

Yes, well on main the bug was non-deterministic.