Iterate over faces instead of vertices, and multithread the logic over
all PBVH nodes. The result is about 3 times faster in my tests;
noticeably snappier for large meshes. Add a new sculpt undo type for
face hide storage (as opposed to vertex storage), and use that only
when the data actually changed, which should reduce memory usage too.
Internally some code is shared with the other visibility operators,
since this operation always affects base mesh faces, most isn't.
Similar to 4e66769ec0
251069ae5b