Blender Git Loki

Multires: Initial Face Sets support

This implements the Sculpt Mode API functions needed for Face Sets and
visibility management for PBVH_GRIDS. No major changes were needed in
the operators and the sculpt mode code. This implementation stores the
face sets in the base mesh, so faces created in higher subdivision
levels can't be modified individually. Also, we are not checking for
multiple face sets per vertex (that can be added in the future), so
relax tools don't work yet. The rest of the features (paint, undo,
visibility operators..) work as expected.

Reviewed By: brecht

Differential Revision: https://developer.blender.org/D7168