The idea is to push both base mesh geometry and PBVH coordinates
so it is possible to undo everything without loosing data which was
not flushed from sculpt session to base mesh.
It is possible do memory optimization to avoid push custom data
layers which are not touched by operator, but before doing that
better to ensure this is a correct and working approach.
Differential Revision: https://developer.blender.org/D7381
b2c2d7b7f1