Blender Git Loki

Add skeletal beginnings of C++ sculpt refactor
design study (note that it's #ifdef'd out).

Basic idea is to put all the sculpt brush code
in a single large template. This template
takes a PBVH adaptor class (of which there
would be three, one per PBVH_XXX type)
as a template argument.

Basically we're using the compiler to generate
three complete copies of every brush implementation.
C++20 concepts are used to validate the pbvh classes.

An example brush implementation:

pbvh->forVertsInRange(
{
.brush = ss->cache->brush,
.radius = radius,
.use_threads = true,
.use_original = false
},

[&offset](auto viter, int node_i, void *userdata) {
//add offset to vertex coordinates

madd_v3_v3fl(viter.co, offset, viter.fade);
},

[](PBVHNode *node, int node_i, void *userdata) {
BKE_pbvh_node_mark_update(node);
});