bug#53636: 29.0.50; face-remapping broken on master

[Eli Zaretskii]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Cc: 53636@debbugs.gnu.org,  tsdh@gnu.org
> Date: Wed, 02 Feb 2022 20:48:42 +0100
> 
> Anyway, what I was thinking of is a really simple solution: Have
> `face-remap-add-relative' loop over all children and remap them, too.

That'd mean looping over all the known faces for each change in
face-remapping-alist, wouldn't it?  Because we don't track which faces
inherit from a given face, we can only tell the reverse: from which
faces a given face inherits.