Blender Git Loki

Fix Face Set operators not modifying sigle poly Face Sets

The face_set_set function which sets a face sets given a vertex index
can ignore all modifications to hidden face sets, so we can skip all
vertex visibility checks outside that function. This makes the code
faster, simpler and fixes multiple bugs.

Reviewed By: jbakker

Differential Revision: https://developer.blender.org/D7122