Add consuming builder methods for more ergonomic Mesh creation

[coreh]

Objective

• This PR aims to make creating meshes a little bit more ergonomic, specifically by removing the need for intermediate mutable variables.

Solution

• We add methods that consume the Mesh and return a mesh with the specified changes, so that meshes can be entirely constructed via builder-style calls, without intermediate variables;
• Methods are flagged with #[must_use] to ensure proper use;
• Examples are updated to use the new methods where applicable. Some examples are kept with the mutating methods so that users can still easily discover them, and also where the new methods wouldn't really be an improvement.

Examples

Before:

let mut mesh = Mesh::new(PrimitiveTopology::TriangleList);
mesh.insert_attribute(Mesh::ATTRIBUTE_POSITION, vs);
mesh.insert_attribute(Mesh::ATTRIBUTE_NORMAL, vns);
mesh.insert_attribute(Mesh::ATTRIBUTE_UV_0, vts);
mesh.set_indices(Some(Indices::U32(tris)));
mesh

After:

Mesh::new(PrimitiveTopology::TriangleList)
    .with_inserted_attribute(Mesh::ATTRIBUTE_POSITION, vs)
    .with_inserted_attribute(Mesh::ATTRIBUTE_NORMAL, vns)
    .with_inserted_attribute(Mesh::ATTRIBUTE_UV_0, vts)
    .with_indices(Some(Indices::U32(tris)))

Before:

let mut cube = Mesh::from(shape::Cube { size: 1.0 });

cube.generate_tangents().unwrap();

PbrBundle {
    mesh: meshes.add(cube),
    ..default()
}

After:

PbrBundle {
    mesh: meshes.add(
        Mesh::from(shape::Cube { size: 1.0 })
            .with_generated_tangents()
            .unwrap(),
    ),
    ..default()
}

---

Changelog

• Added consuming builder methods for more ergonomic Mesh creation: with_inserted_attribute(), with_removed_attribute(), with_indices(), with_duplicated_vertices(), with_computed_flat_normals(), with_generated_tangents(), with_morph_targets(), with_morph_target_names().

[tbillington]

What is your thought process on what to mark #[inline] vs not?

[coreh]

What is your thought process on what to mark #[inline] vs not?

I just copied whatever was on the equivalent mutating method. 😅 If it had #[inline] I also added it, if it didn't, I didn't add it.

I honestly don't feel qualified enough to do otherwise, as I don't fully understand the heuristics the compiler applies, the impact of inlining vs not-inlining, or know how often these are used e.g. in a tight loop or not in real world scenarios.

[alice-i-cecile]

Nice! Simple, but I like the end result.