Add :distant-foreground to faces.

Add :distant-foreground to faces.

* doc/lispref/display.texi (Face Attributes): Document :distant-foreground.

* etc/NEWS: Mention :distant-foreground.

* lisp/faces.el (face-x-resources): Add :distant-foreground.
(region): Use :distant-foreground for gtk and ns.

* src/dispextern.h (lface_attribute_index): Add
LFACE_DISTANT_FOREGROUND_INDEX.

* src/xfaces.c: Declare color_distance.
(QCdistant_foreground): New variable.
(NEAR_SAME_COLOR_THRESHOLD): New define.
(load_color2): New function.
(load_color): Call load_color2.
(load_face_colors): Call load_color2 and if distant-color is specified
calculate distant and use distant-color if colors are near.
(LFACE_DISTANT_FOREGROUND): New define.
(merge_face_ref, Finternal_set_lisp_face_attribute)
(Finternal_get_lisp_face_attribute)
(x_supports_face_attributes_p): Handle distant-foreground similar to
foreground.
(syms_of_xfaces): DEFSYM QCdistant_foreground.