Blender Git Loki

Sculpt: Fair Face Sets operation for Face Set Edit

This implements a mesh fairing algorithm and implements the fair
operations for Face Set edit. This edit operations create a smooth as
possible geometry patch in the area of the selected Face Set.

The mesh fairing algorithm is designed by Brett Fedack for the addon
"Mesh Fairing": https://github.com/fedackb/mesh-fairing, with some
modifications:

- The main fairing function in BKE_mesh_fair.h does not triangulate
the mesh. For the test I did in sculpt mode results are good enough
without triangulating the topology. Depending on the use and the
result quality needed for a particular tool, the mesh can be
triangulate in the affected area before starting fairing.

- Cotangents loop weights are not implemented yet. The idea is to
also expose the vertex and loop weights in a different function in
case a tool needs to set up custom weights.

This algorithm will also be used to solve the limitations of line
project and implement the Lasso Project and Polyline Project tools.
It can also be used in tools in other areas of Blender, like Edit Mode
or future retopology tools.

Reviewed By: brecht

Differential Revision: https://developer.blender.org/D9603