Instead of storing a boolean "update tag" for every vertex, just
recalculate the normals of all the faces and vertices in affected PBVH
nodes. This avoids the overhead of tracking position updates at the
vertex level, and simplifies the normal calculation, since we can
just calculate values for all the normals in a node.
The main way we gain performance is avoiding building a `VectorSet`
for all vertices and faces that need updates in the entire mesh. That
process had to be single threaded, and was a large bottleneck when many
vertices were affected at a time.
That `VectorSet` was necessary for thread safety deduplication of work
though, because neighboring nodes can't calculate the normals of the
same faces or vertices at the same time. (Normals need to be calculated
for all faces connected to moved vertices and all vertices connected to
those faces). In this PR, we only build a set of the *boundary* faces
and vertices. We calculate those in a separate step, which duplicates
work from the non-boundary calculations, but at least it's threadsafe.
I measured normal recalculation timing when sculpting on a 16 million
vertex mesh. The removal of the `vert_bitmap` array also saves 16 MB
of memory.
| Nodes | Affected Vertices | Before (ms) | After (ms) |
| ----- | ------------ | ----------- | ---------- |
| 4 | 15625 | 0.208 | 0.304 |
| 35 | 136281 | 2.98 | 0.988 |
| 117 | 457156 | 15.0 | 3.21 |
| 2455 | 9583782 | 566 | 84.0 |
Pull Request: https://projects.blender.org/blender/blender/pulls/116209
aaa25bc882