fix normals computation for gltf

[mockersf]

Objective

• some gltf files are broken since Revert "Support calculating normals for indexed meshes" (#12716) and add support for calculating smooth normals #13333

thread 'IO Task Pool (2)' panicked at crates/bevy_render/src/mesh/mesh/mod.rs:581:9:
`compute_flat_normals` can't work on indexed geometry. Consider calling either `Mesh::compute_smooth_normals` or `Mesh::duplicate_vertices` followed by `Mesh::compute_flat_normals`.

• test with example custom_gltf_vertex_attribute or gltf_skinned_mesh

Solution

• Call the wrapper function for normals that will either call compute_flat_normals or compute_smooth_normals as appropriate

Testing

• Ran the two examples mentioned above

[alice-i-cecile]

@adithramachandran can I get your review here?

[IceSentry]

Technically the current behaviour is the 0.13 behaviour. Are those examples new since 0.13?

I'm still not a huge fan of normals not being computed the same based on the vertex layout without any warnings to the user, but I guess that's not a big issue.

[rparrett]

Are those examples new since 0.13?

gltf_skinned_mesh has been around since Bevy 0.7, custom_gltf_vertex_attribute since Bevy 0.11.

[IceSentry]

Oh, I know what's going on. In 0.13 it was just always calling duplicate_meshes and it was removed in the PR that computed smooth normals inside compute_flat_normals.

I feel like it would be better to use that instead of now not always computing flat normals.