Blender Git Loki

Fix T80159: Custom Normals Averaging crash after clearing
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