`BM_mesh_triangulate` is used in exporters (when the "Triangulate"
option is ON), the `Triangulate` modifier and currently also in the
`Triangulate` geometry node (even though there are plans to change this,
see !112264)
So in practice, exporters (Alembic/FBX/OBJ/Collada) were breaking
custom normals for game pipelines (unless everything was triangulated
beforehand).
This change builds upon 93c8955a72 (uses the use
`BM_custom_loop_normals_to_vector_layer` /
`BM_custom_loop_normals_from_vector_layer` pair of calls).
In the case of the `Triangulate` modifier, this had its own try at
preserving custom normals in 7d0fcaa69a -- doing very similar
things but as an option -- this is now removed (so it is always done,
which fits into "interpolate custom data if it's there" design that we have
nowadays).
NOTE: the "Triangulate Faces" operator already did the same
Pull Request: https://projects.blender.org/blender/blender/pulls/121871
f3c32a36bc