Blender Git Loki

Fix T74500: Rebuild the Face Sets datalayer after slicing the geometry

Was crashing SculptSession data will not longer be valid if the total
number of polys is modified when rendering the mesh again.
This deletes all face sets in the mesh when slicing the mask. I'll try
to add code to generate a new face set in with faces that are created
when filling the holes, but for now this avoids the crash.

Reviewed By: brecht

Maniphest Tasks: T74500

Differential Revision: https://developer.blender.org/D7049