Currently the face set of every single face is saved for every sculpt undo step.
When only changing the face sets of a small section of the mesh, this can be quite
wasteful. It also makes face sets a special case compare to all other sculpt undo step
types, which makes the whole system more complex and harder to improve.
Fixes T101203.
Reviewed By: Hans Goudey
Differential Revision: https://developer.blender.org/D16224
Ref D16224
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