When baking some data, we create a new Mesh with edits and modifiers applied.
However, in some cases (e.g. when there is no modifier), the returned Mesh is
actually referencing the original one and its data layers. When autosmooth is
enabled we also split the Mesh. However, since the new Mesh is referencing the
original one, although `BKE_mesh_split_faces` is creating new vertices and edges,
the reallocation of the custom data layers is preempted because of the
reference, so adding the new vertices and edges overwrites valid data
To fix this we duplicate referenced layers before splitting the faces.
Reviewed By: brecht
Differential Revision: https://developer.blender.org/D11703
5f9677fe0c