Fix: Face loop select toggle behaviour in paint mode

As pointed out by @pablovazquez the selection behavior
of the face loop selection in paint modes was missing the toggle feature.

The logic goes like this:
If any of the 2 adjacent faces to the selected edge are selected,
deselect instead of select.

There is still an inconsistency which I left in for this patch,
but is up to discussion.
CTRL+ALT+SHIFT click does an actual deselect in paint mode.
In edit mode it does toggle as well, just as ALT+SHIFT click

I can remove that feature if it isn't wanted

Pull Request: https://projects.blender.org/blender/blender/pulls/108753

source/blender/editors/mesh/editface.cc

@@ -517,7 +517,11 @@ void paintface_select_loop(bContext *C, Object *ob, const int mval[2], const boo
   bke::SpanAttributeWriter<bool> select_poly = attributes.lookup_or_add_for_write_span<bool>(
       ".select_poly", ATTR_DOMAIN_FACE);
 
-  select_poly.span.fill_indices(polys_to_select.as_span(), select);
+  /* Toggling behavior. When one of the faces of the picked edge is already selected,
+   * it deselects the loop instead. */
+  const bool select_toggle = select && !(select_poly.span[polys_to_closest_edge[0]] ||
+                                         select_poly.span[polys_to_closest_edge[1]]);
+  select_poly.span.fill_indices(polys_to_select.as_span(), select_toggle);
 
   select_poly.finish();
   paintface_flush_flags(C, ob, true, false);