This prevents UV layer mix up in
MeshBatchCache.cd_used/cd_needed/cd_used_over_time which depends on the
extraction method.
One object's mesh can be accessed with MR_EXTRACT_MESH, another object
that uses the same mesh can use MR_EXTRACT_BMESH based on
(Object.mode & OB_MODE_EDIT), this causes a problem as the edit-mesh
and the mesh aren't always in sync, the custom data layers wont
necessarily match up, causing T77359.
Reviewed by @jbakker, @brecht
Ref D8645
Campbell Barton
33ac3582bb