Sculpting tools are designed to ignore hidden geometry and behave like
hidden geometry does not exist.
When getting the neighbors of a vertex, now this takes into account
hidden geometry to avoid returing neighbors which connected edge is not
visible. This should make corner cases of a lot of tools work properly,
especially when working in low poly meshes when is common to have a
single face loop hidden.
Reviewed By: JacquesLucke
Differential Revision: https://developer.blender.org/D11007
bd76184966