Geometry that was just added to sculpt mode has the SCULPT_FACE_SET_NONE
assigned, so it was hidden by default. By doing this when entering
sculpt mode a new visible face set is created for it, making it easier
to isolate it again if you want to do further tweaking with the sculpt
Also, this also fixes the issue that may happen when changing the mesh
visibility in edit mode. Now visibility changes done outside sculpt mode
are stored in the face sets when entering sculpt mode, so mesh
visibility should stay the same.