This patch adds basic face iterators to the sculpt API. The interface is similar to the existing vertex iterators. It's not C++ (though it does mark private fields in PBVHFaceIter as private if compiling under C++).
Example:
```
PBVHFaceIter fd;
BKE_pbvh_face_iter_begin(pbvh, node, &fd) {
/* Face reference and face index */
PBVHFaceRef face = fd->face;
int face_index = fd->index;
/* Can read and modify hide flag if it exist (it may not) */
if (fd->hide) {
*fd->hide ^= true; /* toggle hide */
}
/* Can read and modify face set if it exists */
if (fd->face_set) {
*fd->face_set = something;
}
/*Can read vertices*/
for (int i=0; i<fd.verts_num; i++) {
float *co = SCULPT_vertex_co_get(ss, fd.verts[i]);
}
}
BKE_pbvh_face_iter_end(&fd);
```