Remove camera from UiBatch (#17663)

# Objective

A `TransparentUI` phase's items all target the same camera so there is
no need to store the current camera entity in `UiBatch` and ending the
current `UiBatch` on camera changes is pointless as the camera doesn't
change.

## Solution

Remove the `camera` fields from `UiBatch`, `UiShadowsBatch` and
`UiTextureSliceBatch`.
Remove the camera changed check from `prepare_uinodes`.

## Testing
The `multiple_windows` and `split_screen` examples both render UI
elements to multiple cameras and can be used to test these changes.

The UI material plugin already didn't store the camera entity per batch
and worked fine without it.

Author: ickshonpe

crates/bevy_ui/src/render/mod.rs

@@ -936,7 +936,6 @@ pub(crate) const QUAD_INDICES: [usize; 6] = [0, 2, 3, 0, 1, 2];
 pub struct UiBatch {
     pub range: Range<u32>,
     pub image: AssetId<Image>,
-    pub camera: Entity,
 }
 
 /// The values here should match the values for the constants in `ui.wgsl`
@@ -1066,8 +1065,6 @@ pub fn prepare_uinodes(
                         || (batch_image_handle != AssetId::default()
                             && extracted_uinode.image != AssetId::default()
                             && batch_image_handle != extracted_uinode.image)
-                        || existing_batch.as_ref().map(|(_, b)| b.camera)
-                            != Some(extracted_uinode.extracted_camera_entity)
                     {
                         if let Some(gpu_image) = gpu_images.get(extracted_uinode.image) {
                             batch_item_index = item_index;
@@ -1076,7 +1073,6 @@ pub fn prepare_uinodes(
                             let new_batch = UiBatch {
                                 range: vertices_index..vertices_index,
                                 image: extracted_uinode.image,
-                                camera: extracted_uinode.extracted_camera_entity,
                             };
 
                             batches.push((item.entity(), new_batch));

crates/bevy_ui/src/render/ui_texture_slice_pipeline.rs

@@ -88,7 +88,6 @@ struct UiTextureSliceVertex {
 pub struct UiTextureSlicerBatch {
     pub range: Range<u32>,
     pub image: AssetId<Image>,
-    pub camera: Entity,
 }
 
 #[derive(Resource)]
@@ -446,8 +445,6 @@ pub fn prepare_ui_slices(
                         || (batch_image_handle != AssetId::default()
                             && texture_slices.image != AssetId::default()
                             && batch_image_handle != texture_slices.image)
-                        || existing_batch.as_ref().map(|(_, b)| b.camera)
-                            != Some(texture_slices.extracted_camera_entity)
                     {
                         if let Some(gpu_image) = gpu_images.get(texture_slices.image) {
                             batch_item_index = item_index;
@@ -457,7 +454,6 @@ pub fn prepare_ui_slices(
                             let new_batch = UiTextureSlicerBatch {
                                 range: vertices_index..vertices_index,
                                 image: texture_slices.image,
-                                camera: texture_slices.extracted_camera_entity,
                             };
 
                             batches.push((item.entity(), new_batch));