Blender Git Loki

Sculpt: Refactor transform code to allow incremental updates

This adds support for incremental updates in the sculpt transform
code. Now tools can define if they need the displacement applied
for the original coordinates or incrementally.

This is needed for features like elastic transform or cloth deformation
target in the transform tool.

No functional changes.

Reviewed By: sergey

Differential Revision: https://developer.blender.org/D9547