bug#65632: 30.0.50; Proposal to improve `faces--attribute-at-point'.

[David Ponce]

Hello,

I noticed that the functions `foreground-color-at-point' and
`background-color-at-point' don't return expected values when the face
at point includes anonymous face or is a nested list of face, for
example.

Here is a simple recipe that demonstrates the issue (emacs -Q):

In scratch buffer eval:
-----------------------

;; Display "TEST" in red, bold, italic on yellow background.
(insert
 (propertize
  "TEST" 'font-lock-face
  '(bold ((:background "yellow") "italic"
          ((foreground-color . "red") underline)))))
TESTnil

;; Then click to move point somewhere on TEST and run
M-: (foreground-color-at-point) RET
> > > result is "black" instead of "red"
M-: (background-color-at-point) RET
> > > result is "white" instead of "yellow"

I propose the attached patch to faces.el to improve things.
The patch introduce a new function `face-attribute-lookup' to lookup
face attribute, that works when face specification is complex like in
above example.  The function `faces--attribute-at-point' is simplified
to use it.

Here is a possible changelog:

        * faces.el: Improve attribute lookup of face at point.
        (face--unnamed-attributes): New constant.
        (face--attribute-unspecified-p)
        (face-attribute-lookup): New functions.
        (faces--attribute-at-point): Use it.  Remove useless argument.
        (foreground-color-at-point)
        (background-color-at-point): Update accordingly.

Thanks

faces-attribute-lookup-V0.patch
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