Sculpt: New Cavity Automasking Mode

Add new cavity automasking mode based on local mesh
curvature.  Cavity masking is a great way to quickly add
detail in crevices and the like.  It's meant to be used
with the Paint brush in color attribute mode.  It does
work with other brushes but the results can be unpredictable.

{F13131497}

The old "dirty mask" operator has been replace with a new
"mask from cavity" operator that shares the same code with
cavity automasking.

Differences from the sculpt-dev implementation:
    * It uses the word "cavity."  When I first implemented
this I wasn't aware
      this feature existed in other software (and other
paint modes in Blender),
      and for reasons that escape me today I initially
decided to call it a concave or
      concavity mask.
    * The cavity factor works a bit differently.  It's
      no longer non-linear and functions as a simple
      scale around 0.5f.
    * Supports custom curves.
    * Supports blurring.

Reviewed By: Julian Kaspar, Jeroen Bakker and Campbell Barton
Differential Revision: https://developer.blender.org/D15122
Ref D15122

tests/python/bl_run_operators.py

@@ -83,7 +83,7 @@ op_blacklist = (
     "object.voxel_remesh",
     "mesh.paint_mask_slice",
     "paint.mask_flood_fill",
-    "sculpt.dirty_mask",
+    "sculpt.mask_from_cavity",
     # TODO: use empty temp dir to avoid behavior depending on local setup.
     "view3d.pastebuffer",
     # Needs active window.