This adds a new Grow/Shrink brush which is similar to the Length
brush in the old hair system.
* It's possible to switch between growing and shrinking by hold
down ctrl and/or by changing the direction enum.
* 3d brush is supported.
* Different brush falloffs are supported.
* Supports scaling curves uniformly or shrinking/extrapolating
them. Extrapolation is linear only in this patch.
* A minimum length settings helps to avoid creating zero-sized curves.
Differential Revision: https://developer.blender.org/D14474
190334b47d