Blender Git Loki

Fix Cloth brush grab artifacts in the affected area

The cloth brush grab mode was creating constraints at 1.0 strength in
the area of the brush where the fade was evaluated to 1. This was causing
stability issues in the simulation and not producing ideal results.
Now the constraint strength is scaled with an empirically found factor.

The values in this patch may require further tweaking after experimenting
a little bit more with them.

Reviewed By: sergey, zeddb

Differential Revision: https://developer.blender.org/D9201