UI batching Fix (bevyengine#9610)

# Objective

Reimplement bevyengine#8793 on top of the recent rendering changes.

## Solution

The batch creation logic is quite convoluted, but I tested it on enough
examples to convince myself that it works.

The initial value of `batch_image_handle` is changed from
`HandleId::Id(Uuid::nil(), u64::MAX)` to `DEFAULT_IMAGE_HANDLE.id()`,
which allowed me to make the if-block simpler I think.

The default image from `DEFAULT_IMAGE_HANDLE` is always inserted into
`UiImageBindGroups` even if it's not used. I tried to add a check so
that it would be only inserted when there is only one batch using the
default image but this crashed.

---

## Changelog
`prepare_uinodes`
* Changed the initial value of `batch_image_handle` to
`DEFAULT_IMAGE_HANDLE.id()`.
* The default image is added to the UI image bind groups before
assembling the batches.
* A new `UiBatch` isn't created when the next `ExtractedUiNode`s image
is set to `DEFAULT_IMAGE_HANDLE` (unless it is the first item in the UI
phase items list).

crates/bevy_ui/src/render/mod.rs

@@ -732,6 +732,7 @@ pub fn queue_uinodes(
         transparent_phase
             .items
             .reserve(extracted_uinodes.uinodes.len());
+
         for (entity, extracted_uinode) in extracted_uinodes.uinodes.iter() {
             transparent_phase.add(TransparentUi {
                 draw_function,
@@ -781,11 +782,6 @@ pub fn prepare_uinodes(
         };
     }
 
-    #[inline]
-    fn is_textured(image: AssetId<Image>) -> bool {
-        image != AssetId::default()
-    }
-
     if let Some(view_binding) = view_uniforms.uniforms.binding() {
         let mut batches: Vec<(Entity, UiBatch)> = Vec::with_capacity(*previous_len);
 
@@ -798,19 +794,21 @@ pub fn prepare_uinodes(
 
         // Vertex buffer index
         let mut index = 0;
-
         for mut ui_phase in &mut phases {
             let mut batch_item_index = 0;
             let mut batch_image_handle = AssetId::invalid();
 
             for item_index in 0..ui_phase.items.len() {
                 let item = &mut ui_phase.items[item_index];
                 if let Some(extracted_uinode) = extracted_uinodes.uinodes.get(&item.entity) {
-                    let mut existing_batch = batches
-                        .last_mut()
-                        .filter(|_| batch_image_handle == extracted_uinode.image);
-
-                    if existing_batch.is_none() {
+                    let mut existing_batch = batches.last_mut();
+
+                    if batch_image_handle == AssetId::invalid()
+                        || existing_batch.is_none()
+                        || (batch_image_handle != AssetId::default()
+                            && extracted_uinode.image != AssetId::default()
+                            && batch_image_handle != extracted_uinode.image)
+                    {
                         if let Some(gpu_image) = gpu_images.get(extracted_uinode.image) {
                             batch_item_index = item_index;
                             batch_image_handle = extracted_uinode.image;
@@ -840,9 +838,32 @@ pub fn prepare_uinodes(
                         } else {
                             continue;
                         }
+                    } else if batch_image_handle == AssetId::default()
+                        && extracted_uinode.image != AssetId::default()
+                    {
+                        if let Some(gpu_image) = gpu_images.get(extracted_uinode.image) {
+                            batch_image_handle = extracted_uinode.image;
+                            existing_batch.as_mut().unwrap().1.image = extracted_uinode.image;
+
+                            image_bind_groups
+                                .values
+                                .entry(batch_image_handle)
+                                .or_insert_with(|| {
+                                    render_device.create_bind_group(
+                                        "ui_material_bind_group",
+                                        &ui_pipeline.image_layout,
+                                        &BindGroupEntries::sequential((
+                                            &gpu_image.texture_view,
+                                            &gpu_image.sampler,
+                                        )),
+                                    )
+                                });
+                        } else {
+                            continue;
+                        }
                     }
 
-                    let mode = if is_textured(extracted_uinode.image) {
+                    let mode = if extracted_uinode.image != AssetId::default() {
                         TEXTURED_QUAD
                     } else {
                         UNTEXTURED_QUAD