The implementation follows the logic of the face set brush for the
faces and grids type of PBVH, with the similar weak part of iterating
over entire mesh at the start of a stroke.
The difference in the behavior is that face needs to be fully covered
by the brush in order to have face set assigned to it (while for the
other types of the PBVH face gets assigned its face set if any of its
vertices are covered). The main reason for this is that this seems to
avoid boundaries being too wiggly.
The auto-masking is not fully integrated into this brush yet. Doing so
is possible, but seems to require deeper re-considerations of the way
how automasking accesses original data and how it is fetched from an
undo node. It worth noting that auto-masking is something that needs
to be looked into for all brushes to make it supported in the dynamic
topology mode.
Pull Request: https://projects.blender.org/blender/blender/pulls/113357
4e87b3004b