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Sculpt: mesh abstraction API

These functions make possible porting the tools from the sculpt branch, making them compatible with PBVH_FACES and PBVH_BMESH without duplicating the code. They can also help to simplify some existing code.

These functions should not be used when working with PBVH_GRIDS data in SculptSession. PBVH_GRIDS needs to be removed from the sculpt code and converted to PBVH_FACES to be compatible with this API.

Reviewed By: brecht

Differential Revision: https://developer.blender.org/D5352