Replace remaining uses of &T, Changed<T> with Ref in UI system queries

[ickshonpe]

Objective

Replace Query<&T, Changed<T>> style queries with the more efficient Query<Ref<T>> form in two of the UI systems.

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Changelog

Replaced use of Changed with Ref in queries in the ui_layout_system and calc_bounds UI systems.

[alice-i-cecile]

Much clearer!

[vultix]

I'm a bit confused by this - how is Query<Ref<T>> more efficient than Query<&T, Changed<T>>?

I'd assumed that using Changed<T> would do archetype filtering, leading to reduced iterations. With Ref<T> you need to iterate through all entities containing T, even if they are only changed sparsely, right?

[alice-i-cecile]

Changed<T> doesn't operate on an archetype level currently: only individual entities.

[inodentry]

@alice-i-cecile I have a question. Didn't we at some point talk about possibly tracking that information at the archetype level in the ECS in the future, to allow for more efficient change detection? So that query filters can actually result in an efficiency improvement by only accessing changed entities?

This PR seems counterproductive. If we ever do that, we would have to change it back.

Also, performance aside, I really disagree that using Ref is always necessarily clearer. If the intent is to only access changed entities, then the query filter makes that clear. If the intent is to access all entities, but be able to do different logic if entities were changed or not, then Ref is clearer.

I know this PR was merged long ago, and me expressing an opinion now probably doesn't matter much, but this really feels like the wrong direction to me.

[alice-i-cecile]

That's a valid position, and yeah, we should be able to have multiple layers of change detection for faster operation in the future.