The normals were not allocated before the brush code starts, which is
necessary to store the initial normals in the undo step, among other
things. Previously normals were calculated as part of building the PBVH.
That doesn't make sense anymore because that process shouldn't change
the mesh. For now just add the recalculation in a code path called by
`BKE_sculpt_update_object_for_edit`, called at the beginning of each
stroke, and other places.
a19760678e