Sculpt: Face iterator API

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);
```

Reviewed By: Brecht Von Lommen and Hans Goudey
Differential Revision: https://developer.blender.org/D16225
Ref D16225

.clang-format

@@ -162,6 +162,7 @@ PenaltyBreakString: 1000000
 ForEachMacros:
   - BEGIN_ANIMFILTER_SUBCHANNELS
   - BKE_pbvh_vertex_iter_begin
+  - BKE_pbvh_face_iter_begin
   - BLI_FOREACH_SPARSE_RANGE
   - BLI_SMALLSTACK_ITER_BEGIN
   - BMO_ITER