Blender Git Loki

Dyntopo now updates the existing pbvh on undo instead of building
a new one from scratch, an operation that can be slow despite being
threaded.

PBVH building is a memory bound operation (not just on the CPU side
either, remember the draw buffers have to be fully regenerated too).
Incrementally updating it this way is enormously faster (about as fast
as non-dyntopo undo).

The downside is we don't have the convienience of users regularly
building the pbvh from scratch anymore. Dyntopo does try to
join empty PBVH nodes (which happens after every stroke), but
that's not a complete substitute for a decent tree balancer.
That's on the todo list.