Blender Git Loki

Fix T80008: Smooth brush not deforming mesh boundaries correctly

In 2.83 and previous versions there was a bug that was causing boundary
vertices to be detected incorrectly that was preventing the smooth brush
to work on boundaries if there was a pole on them.
In 2.90 the boundary vertex detection was fixed, but it was still using a
simplified version of the algorithm without any boundary smoothing. This
patch implements a similar smoothing algorithm to what I think it was
the intention of 2.83 and previous versions, but working correctly.

Reviewed By: sergey

Maniphest Tasks: T80008

Differential Revision: https://developer.blender.org/D8680