Fix Mesh allocator bug and reduce Mesh data copies by two (#15566)

# Objective

- First step towards #15558

## Solution

- Rename `get_vertex_buffer_data` to `create_packed_vertex_buffer_data`
to make it clear that it is not "free" and actually allocates
- Compute length analytically for preallocation instead of creating the
buffer to get its length and immediately discard it
- Use existing vertex attribute size calculation method to reduce code
duplication
- Fix a bug where mesh index data was being replaced by unnecessarily
newly created mesh vertex data in some cases
- Overall reduces mesh copies by two. We still have plenty to go, but
these were the easy ones.

## Testing

- I ran 3d_scene, lighting, and many_cubes, they look fine.
- Benchmarks would be nice, but this is very obviously a win in perf and
correctness.

---

## Migration Guide

- `Mesh::create_packed_vertex_buffer_data` has been renamed
`Mesh::create_packed_vertex_buffer_data` to reflect the fact that it
copies data and allocates.

## Showcase

- look mom, less copies

crates/bevy_pbr/src/meshlet/from_mesh.rs

@@ -30,7 +30,7 @@ impl MeshletMesh {
         let indices = validate_input_mesh(mesh)?;
 
         // Split the mesh into an initial list of meshlets (LOD 0)
-        let vertex_buffer = mesh.get_vertex_buffer_data();
+        let vertex_buffer = mesh.create_packed_vertex_buffer_data();
         let vertex_stride = mesh.get_vertex_size() as usize;
         let vertices = VertexDataAdapter::new(&vertex_buffer, vertex_stride, 0).unwrap();
         let mut meshlets = compute_meshlets(&indices, &vertices);

crates/bevy_render/src/mesh/allocator.rs

@@ -427,7 +427,7 @@ impl MeshAllocator {
             if self.general_vertex_slabs_supported {
                 self.allocate(
                     mesh_id,
-                    mesh.get_vertex_buffer_data().len() as u64,
+                    mesh.get_vertex_size() * mesh.count_vertices() as u64,
                     vertex_element_layout,
                     &mut slabs_to_grow,
                     mesh_allocator_settings,
@@ -474,12 +474,11 @@ impl MeshAllocator {
         let Some(&slab_id) = self.mesh_id_to_vertex_slab.get(mesh_id) else {
             return;
         };
-        let vertex_data = mesh.get_vertex_buffer_data();
+        let vertex_data = mesh.create_packed_vertex_buffer_data();
 
         // Call the generic function.
         self.copy_element_data(
             mesh_id,
-            mesh,
             &vertex_data,
             BufferUsages::VERTEX,
             slab_id,
@@ -507,7 +506,6 @@ impl MeshAllocator {
         // Call the generic function.
         self.copy_element_data(
             mesh_id,
-            mesh,
             index_data,
             BufferUsages::INDEX,
             slab_id,
@@ -521,7 +519,6 @@ impl MeshAllocator {
     fn copy_element_data(
         &mut self,
         mesh_id: &AssetId<Mesh>,
-        mesh: &Mesh,
         data: &[u8],
         buffer_usages: BufferUsages,
         slab_id: SlabId,
@@ -567,7 +564,7 @@ impl MeshAllocator {
                             slab_id,
                             buffer_usages_to_str(buffer_usages)
                         )),
-                        contents: &mesh.get_vertex_buffer_data(),
+                        contents: data,
                         usage: buffer_usages | BufferUsages::COPY_DST,
                     },
                 ));

crates/bevy_render/src/mesh/mesh/mod.rs

@@ -458,13 +458,8 @@ impl Mesh {
     ///
     /// If the vertex attributes have different lengths, they are all truncated to
     /// the length of the smallest.
-    pub fn get_vertex_buffer_data(&self) -> Vec<u8> {
-        let mut vertex_size = 0;
-        for attribute_data in self.attributes.values() {
-            let vertex_format = attribute_data.attribute.format;
-            vertex_size += vertex_format.get_size() as usize;
-        }
-
+    pub fn create_packed_vertex_buffer_data(&self) -> Vec<u8> {
+        let vertex_size = self.get_vertex_size() as usize;
         let vertex_count = self.count_vertices();
         let mut attributes_interleaved_buffer = vec![0; vertex_count * vertex_size];
         // bundle into interleaved buffers