Addresses #121240.
Instead of allocating a new array and recalculating mesh triangulation
every time the user enters sculpt mode, reuse the mesh's existing cache.
Currently in order to avoid recalculating triangulation on every brush update
(which would typically be necessary because the triangulation direction
depends on vertex positions), add a mechanism to "freeze" the cache to
skip recalculations until the user exits sculpt mode. That even avoids
recalculation if vertex positions aren't affected. This is necessary because
we can't use the cache in a dirty state; tagging the cache dirty frees the
triangulation array.
Removing the duplicate triangles array reduces memory usage by 384 MB
in a 16 million vertex sculpt, and makes entering sculpt mode 125ms faster
(tested on a Ryzen 7840u).
In the long term, I hope we find a different solution that's a bit more
transparent and hopefully more integrated with the caching system
in general. In the meantime, this is a relatively safe low impact change
that helps document the needs for such a system anyway.
Pull Request: https://projects.blender.org/blender/blender/pulls/123638
59d6eae116