Eliminate the "display faces"; store GCs in the "computed faces".

Eliminate the "display faces"; store GCs in the "computed faces".
(face_vector, nfaces, nfaces_allocated): Vars deleted.
(free_frame_faces): Free GCs in the computed faces.
Free the computed faces themselves.
(clear_face_cache): Renamed from clear_face_vector
and completely rewritten to free GCs in the computed faces.
(get_cached_face): Function deleted.
(build_face): Function deleted; code moved into intern_face.
(intern_face): Don't treat default and modeline faces specially.
(recompute_basic_faces): Call intern_face, not build_face.

(intern_face, clear_face_vector, load_font, unload_font)
(load_color, free_frame_faces, recompute_basic_faces): Use FRAME_X_DISPLAY.