bug#29353: [PATCH] Add window divider faces to NS (bug#29353)

[Keith David Bershatsky]

Thank you, Alan, for bringing this feature to Emacs on the OSX/MacOS platforms.

The only issue I observed with the patch applied is when a user selects a 
divider width of 3 for right and bottom.  In that situation, the bottom divider 
is entirely one color -- window-divider face.  And, the right divider is 2 
pixels in the window-divider-first-pixel face and 1 pixel in the 
window-divider-last-pixel face.  When there are exactly 3 pixels, both dividers 
should have the rainbow of all three available colors in the applicable order.

The resolution on my screen is not the greatest in the world, but it looks like 
all other width variations are working properly.  I.e., 1 and 2 pixel widths 
properly have just the window-divider color (since there is agreeably no room 
for both the first/last pixel colors), and 4 pixels in width and above have the 
rainbow effect.

(face-spec-set 'default '((t :background "black" :foreground "red")))

(face-spec-set 'window-divider-first-pixel '((t :foreground "red")))

(face-spec-set 'window-divider '((t :foreground "DarkBlue")))

(face-spec-set 'window-divider-last-pixel '((t :foreground "magenta")))

(with-selected-frame
  (make-frame '((right-divider-width . 3) (bottom-divider-width . 3)))
  (split-window-horizontally))

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DATE:  [11-19-2017 03:53:13] <19 Nov 2017 11:53:13 +0000>
FROM:  Alan Third <alan@idiocy.org>
> 
> * src/nsterm.m (ns_draw_window_divider): Use
> window-divider-first-pixel and window-divider-last-pixel faces.
> ---
> * * *