Compute texture slices after layout (#12533)

# Objective

Whenever a nodes size gets changed, its texture slices get updated a
frame later. This results in visual glitches when animating the size of
a node with a texture slice. See this video:

[Screencast from 17-03-24
14:53:13.webm](https://github.com/bevyengine/bevy/assets/46689298/64e711f7-a1ec-41e3-b119-dc8d7e1a7669)


## Solution

Compute texture slices after the layout system has finished.

crates/bevy_ui/src/lib.rs

@@ -149,17 +149,15 @@ impl Plugin for UiPlugin {
  // They run independently since `widget::image_node_system` will only ever observe
  // its own UiImage, and `widget::text_system` & `bevy_text::update_text2d_layout`
  // will never modify a pre-existing `Image` asset.
+ widget::update_image_content_size_system
+ .before(UiSystem::Layout)
+ .in_set(AmbiguousWithTextSystem)
+ .in_set(AmbiguousWithUpdateText2DLayout),
  (
- widget::update_image_content_size_system
- .before(UiSystem::Layout)
- .in_set(AmbiguousWithTextSystem)
- .in_set(AmbiguousWithUpdateText2DLayout),
- (
- texture_slice::compute_slices_on_asset_event,
- texture_slice::compute_slices_on_image_change,
- ),
+ texture_slice::compute_slices_on_asset_event,
+ texture_slice::compute_slices_on_image_change,
  )
- .chain(),
+ .after(UiSystem::Layout),
  ),
  );