Blender Git Loki

Fix T79965: mesh.transform() modifies referenced layers

Originally was noticed when transforming mesh created by
object.to_mesh() from an object without modifier, in which case the
result references CustomData layers used by the object itself.

The issue goes a bit deeper: mesh.transform() should never modify
referenced layers, hence it should duplicate referenced layers.

This fix changes one specific aspect of the reported behavior. The
case where vertices coordinates are modified manually will still have
affect on the source mesh (as no referenced CustomData layers are being
duplicated). Proper fix for this case is not yet clear to me.

Differential Revision: https://developer.blender.org/D8939