Blender Git Loki

Fix repeated Face Sets IDs when joining meshes

As Face Sets IDs start from 0 and increase by 1 each time a new face set
is created in a mesh, when joining multiple meshes it could happen that
the same ID is used by several unrelated areas in multiple objects. This
checks the Face Sets IDs when joining meshes and ensures that they are
not repeated between different objects when joining them, so in the
resulting mesh all previous face sets will have different IDs.

Reviewed By: sergey

Differential Revision: https://developer.blender.org/D8224