This adds support for X/Y/Z symmetry for all brushes in curves
sculpt mode. In theory this can be extended to support radial
symmetry, but that's not part of this patch.
It works by essentially applying a brush stroke multiple with
different transforms. This is similiar to how symmetry works in
mesh sculpt mode, but is quite different from how it worked in
the old hair system (there it tried to find matching hair strands
on both sides of the surface; if none was found, symmetry did
not work).
Differential Revision: https://developer.blender.org/D14795
7dc94155f6