SCULPT_FACE_SET_NONE default value is 0 and it is rendered hidden, so
the invert sign operation to show it was not working. Now the show all
function sets this face set to ID 1 before setting its sign.
I also refactored this check in gpu_buffers.
Not related to the reported issue, but the mesh in attached contains non
manifold geometry with hidden loose vertices, so the visibility state
was not syncing correctly to those vertices. Now the toggle operators
checks the current visibility only on the face sets, so no manifold
vertices are ignored (as they are in the rest of operations in sculpt