Add compute_*_normals benchmarks (#18648)

# Objective

Benchmark current `compute_*_normals` methods to get a baseline as
requested in
https://github.com/bevyengine/bevy/pull/18552#issuecomment-2764875143

## Solution

Since the change to the default smooth normals method will definitely
cause a regression, but the previous method will remain as an option, I
added two technically-redundant benchmarks but with different names:
`smooth_normals` for whatever default weighting method is used, and
`face_weighted_normals` to benchmark the area-weighted method regardless
of what the default is. Then I'm adding `angle_weighted_normals` in
#18552. I also added `flat_normals` for completeness.

benches/benches/bevy_render/compute_normals.rs

@@ -0,0 +1,96 @@
+use core::hint::black_box;
+
+use criterion::{criterion_group, Criterion};
+use rand::random;
+use std::time::{Duration, Instant};
+
+use bevy_render::{
+    mesh::{Indices, Mesh, PrimitiveTopology},
+    render_asset::RenderAssetUsages,
+};
+
+const GRID_SIZE: usize = 256;
+
+fn compute_normals(c: &mut Criterion) {
+    let indices = Indices::U32(
+        (0..GRID_SIZE - 1)
+            .flat_map(|i| std::iter::repeat(i).zip(0..GRID_SIZE - 1))
+            .flat_map(|(i, j)| {
+                let tl = ((GRID_SIZE * j) + i) as u32;
+                let tr = tl + 1;
+                let bl = ((GRID_SIZE * (j + 1)) + i) as u32;
+                let br = bl + 1;
+                [tl, bl, tr, tr, bl, br]
+            })
+            .collect(),
+    );
+
+    let new_mesh = || {
+        let positions = (0..GRID_SIZE)
+            .flat_map(|i| std::iter::repeat(i).zip(0..GRID_SIZE))
+            .map(|(i, j)| [i as f32, j as f32, random::<f32>()])
+            .collect::<Vec<_>>();
+        Mesh::new(
+            PrimitiveTopology::TriangleList,
+            RenderAssetUsages::MAIN_WORLD,
+        )
+        .with_inserted_attribute(Mesh::ATTRIBUTE_POSITION, positions)
+        .with_inserted_indices(indices.clone())
+    };
+
+    c.bench_function("smooth_normals", |b| {
+        b.iter_custom(|iters| {
+            let mut total = Duration::default();
+            for _ in 0..iters {
+                let mut mesh = new_mesh();
+                black_box(mesh.attribute(Mesh::ATTRIBUTE_NORMAL));
+                let start = Instant::now();
+                mesh.compute_smooth_normals();
+                let end = Instant::now();
+                black_box(mesh.attribute(Mesh::ATTRIBUTE_NORMAL));
+                total += end.duration_since(start);
+            }
+            total
+        });
+    });
+
+    c.bench_function("face_weighted_normals", |b| {
+        b.iter_custom(|iters| {
+            let mut total = Duration::default();
+            for _ in 0..iters {
+                let mut mesh = new_mesh();
+                black_box(mesh.attribute(Mesh::ATTRIBUTE_NORMAL));
+                let start = Instant::now();
+                mesh.compute_smooth_normals();
+                let end = Instant::now();
+                black_box(mesh.attribute(Mesh::ATTRIBUTE_NORMAL));
+                total += end.duration_since(start);
+            }
+            total
+        });
+    });
+
+    let new_mesh = || {
+        new_mesh()
+            .with_duplicated_vertices()
+            .with_computed_flat_normals()
+    };
+
+    c.bench_function("flat_normals", |b| {
+        b.iter_custom(|iters| {
+            let mut total = Duration::default();
+            for _ in 0..iters {
+                let mut mesh = new_mesh();
+                black_box(mesh.attribute(Mesh::ATTRIBUTE_NORMAL));
+                let start = Instant::now();
+                mesh.compute_flat_normals();
+                let end = Instant::now();
+                black_box(mesh.attribute(Mesh::ATTRIBUTE_NORMAL));
+                total += end.duration_since(start);
+            }
+            total
+        });
+    });
+}
+
+criterion_group!(benches, compute_normals);

benches/benches/bevy_render/main.rs

@@ -1,6 +1,11 @@
 use criterion::criterion_main;
 
+mod compute_normals;
 mod render_layers;
 mod torus;
 
-criterion_main!(render_layers::benches, torus::benches);
+criterion_main!(
+    render_layers::benches,
+    compute_normals::benches,
+    torus::benches
+);