This commit introduces a number of changes to make PBVH and related
attribute lifecycle more predictable.
A common method, `BKE_sculptsession_free_pbvh` is introduced to manage
freeing related resources that are stored in `SculptSession`
Prior to this commit, the `active_vert_` attribute was only ever
changed when a raycast successfully hit the mesh. This commit changes
the behavior to not store a stale reference in the following cases:
* When dyntopo is enabled or disabled
* When a mesh with the subdivision modifier is subdivided further
* When the sculpt level of a subdiv modifier is changed
* When the subdivision modifier is removed from a mesh
Pull Request: https://projects.blender.org/blender/blender/pulls/126341
c9a69f70d9