Before 111e586424, the normals were written back to the mesh
from the values stored in the undo step. Now though, due to caching of
normals and better const correct-ness, this is not so simple or safe.
That still may be a nice optimization to apply in the future, but for
now the simplicity of just recalculating them is much more feasible.
Since the recalculation is localized to the brush action, performance
should be okay. And maybe normals won't have to be stored in undo steps
in the future as well, since they're derived data.
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