RenderUiSystem::ExtractTextureSlice (#15332)

# Objective

Fixes #15330

## Solution
1. Add an `ExtractTextureSlice` variant to `RenderUiSystem`.
2. Add `RenderUiSystem::ExtractTextureSlice` to the `ExtractSchedule`
between `ExtractImages` and `ExtractBorders`.
3. Add `extract_ui_texture_slices` to the new `ExtractTextureSlice`
system set.

Which results in texture slice nodes being extracted before borders. No
more z-fighting, borders will always be drawn on top of texture-sliced
images.

crates/bevy_ui/src/render/mod.rs

@@ -70,6 +70,7 @@ pub const UI_SHADER_HANDLE: Handle<Shader> = Handle::weak_from_u128(130128470471
 pub enum RenderUiSystem {
     ExtractBackgrounds,
     ExtractImages,
+    ExtractTextureSlice,
     ExtractBorders,
     ExtractText,
 }
@@ -95,6 +96,7 @@ pub fn build_ui_render(app: &mut App) {
             (
                 RenderUiSystem::ExtractBackgrounds,
                 RenderUiSystem::ExtractImages,
+                RenderUiSystem::ExtractTextureSlice,
                 RenderUiSystem::ExtractBorders,
                 RenderUiSystem::ExtractText,
             )

crates/bevy_ui/src/render/ui_texture_slice_pipeline.rs

@@ -53,7 +53,7 @@ impl Plugin for UiTextureSlicerPlugin {
                 .init_resource::<SpecializedRenderPipelines<UiTextureSlicePipeline>>()
                 .add_systems(
                     ExtractSchedule,
-                    extract_ui_texture_slices.after(extract_uinode_images),
+                    extract_ui_texture_slices.in_set(RenderUiSystem::ExtractTextureSlice),
                 )
                 .add_systems(
                     Render,