This PR adds a new operator: `PAINT_OT_visibility_edit` to support
iteratively expanding or shrinking the visibility of a mesh, similar to
the *Grow / Shrink Mask* actions and the *Grow / Shrink Face Set*
options. This operator is exposed via two new entries in the *Sculpt*
toolbar entry as *Show More* and *Show Less* and have also been
assigned to Page Up and Page Down in the default Blender keybinds for
Sculpt Mode.
### Technical Details
Each of the PBVH types is solved slightly differently, though the
general principle for each is as follows:
1. Make a copy of the current mesh visibility state
2. Iterate over elements (faces & corners if available, otherwise
vertices) to look at adjacency information
3. Apply appropriate visibility change to vertices
4. Sync face visibility
### Limitations
* Currently, like all other operators in the `paint_hide.cc` file. This
new operator is limited to Sculpt mode only.
Based off of [this](https://blender.community/c/rightclickselect/pz4y/)
RCS request.
Pull Request: https://projects.blender.org/blender/blender/pulls/120282
1ab3fffc1d