This removes all multires code from the smooth brush and replaces it
with a new implementation that uses the sculpt mesh API. This makes the
code much easier to understand and maintain.

We could unify all the smooth brushes into a single implementation, but
mesh smoothing has code to deal with open boundaries that I don't want
to remove yet.

Most bugs and artifacts related to access neighbor grids should be fixed
(like T71667). This also fixes performance bugs such as T70689, that were
related to grid stitching after applying the brush.