Blender Git Loki

Sculpt Undo: Refactor Geometry undo nodes

Made it so there is a single UNDO node in the list which has
both original and modified mesh state.

Makes it easier to achieve "interleaved" undo nodes stored in
the undo step (as opposite of either storing geometry or other
data).

Should be no functional changes, just preparing for an upcoming
work to support undo of operation like Apply Base.

Differential Revision: https://developer.blender.org/D7290