Adding new variable for face-list in internal-make-lisp-face.

[Keith David Bershatsky]

In implementing feature requests crosshairs (17684) and multiple fake cursors 
(22873), I thought it would be nifty to identify the background color (if 
applicable) associated with a tab ('\t') glyph code in the 
buffer-display-table.  In doing so, I discovered that the Lisp function 
face-list is way too slow.  Even after I ported the face-list function over to 
C, it was still way too slow.  So, I tried my luck at adding a new variable in 
xfaces.c for the face-list and am setting it from within the function 
internal-make-lisp-face, like so:

    Vface_list = Fcons (face, Vface_list);

and, it is done within the same section where we already have:

      Vface_new_frame_defaults = Fcons (Fcons (face, global_lface),
        Vface_new_frame_defaults);

Accessing the tab face in the face-list variable works just fine UNTIL I 
manually `eval-defun` on the `defface` for the tab face to change its 
background color.  Thereafter, when I try to get information about that tab 
face in the face-list from within xdisp.c, I get:

Error during redisplay: (my-custom-face-attribute #<EMACS BUG: INVALID DATATYPE 
(MISC 0xc9ff) Save your buffers immediately and please report this bug> 
:background [frame information] default) signaled (error "Invalid face" #<EMACS 
BUG: INVALID DATATYPE (MISC 0xc9ff) Save your buffers immediately and please 
report this bug>)

Here is the gdb breakpoint information when I try to access that face (in the 
face-list variable) after manually changing the background color with 
eval-defun -- I get INVALID_LISP_OBJECT:

Breakpoint 3, Fmc_face_attribute (face=..., attribute=..., frame=..., 
    inherit=...) at xdisp.c:1119
1119      Lisp_Object value = Finternal_get_lisp_face_attribute (face, 
attribute, frame);
(gdb) pp face
#<INVALID_LISP_OBJECT 0x10013c199>

How can I properly implement a new face-list variable so that the lisp object 
will be valid after manually running `eval-defun` on a defface?

Thanks,

Keith