Blender Git Loki

* Implemented bounday/face set corner handling.
* New function SCULPT_vertex_is_corner, similar to
SCULPT_vertex_is_boundary it takes argument to
check face sets. PBVH_FACES/GRIDS version is
incomplete. It returns a bitmask of whether
the vert is a boundary corner and/or a face
set one.
* PBVH_BMESH uses a somewhat more expensive
calculation to detect corners of face set islands by
edge angle. This is currently not done for boundary
corners.

Corner pinning now happens in:

* The internal smoother dyntopo uses for stability reasons.
* SCULPT_vertex_neighbor_average_interior.
* Topology rake.
* Dyntopo collapse.

Note that DynTopo always pins face set corners
but everything else only does so if preserve face
sets is on.