7aa0be4b32
In some cases, the normal edit modifier calculated the normals on one
mesh with the "ensure" functions, then copied the mesh and retrieved
the layers "for write" on the copy. Since 59343ee162, normal
layers are never copied, and normals are allocated with malloc instead
of calloc, so the mutable memory was uninitialized.
Fix by calculating normals on the correct mesh, and also add a warning
to the "for write" functions in the header.