custom split normals data Clearing custom split normals would get rid of the CD_CUSTOMLOOPNORMAL layer - but editing data `lnor_spacearr` would be kept. Adding a CD_CUSTOMLOOPNORMAL layer (if none exists yet) should be done in `edbm_average_normals_exec` / `BKE_editmesh_lnorspace_update` / `BM_lnorspace_update` / `BM_lnorspacearr_store`. The thing is that if the editing data `lnor_spacearr` would still be valid after `Clear Custom Split Normals Data`, blender would happily call `BM_lnorspace_rebuild` instead. Doing that without a CD_CUSTOMLOOPNORMAL layer is asking for trouble. Now clear lnor_spacearr on `Clear Custom Split Normals Data` as well. Thx @mont29 for feedback here. Maniphest Tasks: T80159 Differential Revision: https://developer.blender.org/D8730