Current UiStack based extraction potentially leads to poor batching

## Bevy version
a3baf2ae869611a84cf1bc062c498ea08128cbd9

## What you did
`cargo run --profile stress-test --features trace_tracy --example many_buttons`

## What went wrong
Issuing 24,000+ draw calls when it could be issuing 3.

The construction of `UiStack` forces siblings of a similar depth in the hierarchy to be non-adjacent if any of them have children. This also forces their descendants to not be adjacent, which is breaking large scale batching.

https://github.com/bevyengine/bevy/blob/a3baf2ae869611a84cf1bc062c498ea08128cbd9/crates/bevy_ui/src/stack.rs#L93-L102