bug#35452: Line number faces should check for remapping of the default face

[Eli Zaretskii <eliz@gnu.org>]

> From: Dario Gjorgjevski <dario.gjorgjevski@gmail.com>
> Date: Sat, 27 Apr 2019 14:52:10 +0200
> 
> Currently, the line number faces do not check for remapping of the
> default face and use its attributes directly.  In the default
> configuration, this has no adverse effects since the `line-number' face
> inherits from `default' explicitly, so any remapping is considered there.
> 
> However, there is no need to have `line-number' inherit from `default'
> explicitly since it already merges DEFAULT_FACE_ID.  Instead, we can
> check for remapping of DEFAULT_FACE_ID prior to merging the faces.
> 
> The patch shown below accomplishes that.

Thanks, but I don't think I understand the advantages of this approach
vs the current one.  Concretely, why would we want not to inherit
from the 'default' face?

Also, doesn't your change force the line-number face to change
together with 'default', even if the user defines the face to not
inherit from 'default'?  With the current code, users are free to
define the face without inheritance, and that will stop update the
line-number face together with 'default', e.g. when the user enlarges
the default face's font or makes it smaller.  With your proposal, the
size changes in 'default' will always be propagated to line-number,
right?

And finally, if we do make the proposed change, shouldn't we stop
inheriting from 'default' at the same time?