Blender Git Loki

Fix Cloth Brush grab deformation mode

The grab mode was not correctly implemented, so the way it was working
was confusing for users.
- Grab delta was calculated in increments from the last stroke position, so it did not match the behavior of a grab brush. I refactored the grab delta calculation to make this change more explicit.
- Grab displacement was not calculated from the original coordinates
- Grab was using an incorrect strength

Grab is now setting the position of the affected vertices directly and
the constraints solve the rest of the cloth. I also tried to implement
an alternative version based on applying forces to move the vertices to
the grab position, but I think this is more controllable and the grab
falloff can be adjusted by tweaking the simulation falloff.

Reviewed By: sergey

Differential Revision: https://developer.blender.org/D7756