Mesh validate: add an option to not clean temp/cache CDLayers.

Mesh validate: add an option to not clean temp/cache CDLayers.

This is mandatory for incoming custom normal imports from io scripts, because
often geometry here is corrupted, so we need to call mesh.validate() to clean it up.

Issue is, we cannot set custom normals before geometry is clean, so we need to store
temporary plain loop normals in a CD_NORMAL layer, validate, and then set custom normals.
So we need a way to prevent 'temp' lnors to be freed by validate.