Blender Git Loki

Sculpt: New Layer Brush

The Layer brush was in Blender before 2.81, when the sculpt API was
introduced. It had a huge amount of bugs and glitches which made it
almost unusable for anything but the most trivial cases. Also, it needed
some hacks in the code just to support the persistent base.

The brush was completely rewritten using the Sculpt API. It fulfills the
same use case as the old one, but it has:
- All previous artifacts fixed
- Simpler code
- Persistent base now works with multires thanks to the sculpt API
- Small cursor widget to preview the layer height
- More controllable and smoother strength and deformation
- More correct masking support
- More predictable invert support. When using persistent base, the brush invert mode resets to layer height 0, instead of jumping from +1 to -1. The brush can still be inverted in the brush direction property.

Reviewed By: jbakker

Differential Revision: https://developer.blender.org/D7147