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