Blender Git Loki

Move layer displacements from SculptUndoNode to PBVHNode

* This doesn't make much difference for regular mesh/multires
sculpting, but for dynamic topology sculpting the undo stack isn't
split up by PBVH nodes, so it's more convenient to store the layer
data in PBVH nodes.

* Note that the life cycle of the layer displacement data is
unchanged -- it's only valid during a stroke with the layer brush,
gets free'd when the undo step ends.