Fix face merging when some have :extend non-nil and some are inherited

Fix face merging when some have :extend non-nil and some are inherited

* src/xfaces.c (face_inherited_attr): New function.
(merge_named_face): Call 'face_inherited_attr' when testing
whether a face that inherits from another fits the filtering
criteria specified by ATTR_FILTER.
(merge_face_vectors): Revert the changes made in this function
for filtering by ATTR_FILTER, and remove that argument as
well.  These tests are now completely done by the caller, see
'merge_named_face'.  (Bug#37774)