Prior to 0df133559c, the `active_vert_` property was only ever
invalidated if the entire `SculptSession` was refreshed, even if the
cursor moved off of the mesh or if the underlying mesh type changed.
The Sculpt Expand operator was dependent on this value not being cleared
to still behave in a reasonable way from a user perspective
This commit introduces a new variable, `last_active_vert_` and
corresponding accessor methods to `SculptSession` to specifically fix
this usecase. When the active vert is cleared, if it had a valid value,
we store it in the new `last_active_vert_` variable.
Pull Request: https://projects.blender.org/blender/blender/pulls/127433
ab5ce3061a