Fix image colors on MacOS (#141)

Fixes #54.

Adds a custom option 'powerline-image-apple-rgb', which should be set
to 't' to enable the workaround. When the option is set, powerline's
XPM images are generated using equivalent Apple RGB colors, instead of
the regular sRGB colors.

This is not needed on the Emacs-mac port, or when Apple RGB colors are
used by setting 'ns-use-srgb-colorspace' to nil.

powerline-separators.el

@@ -27,10 +27,25 @@ COLOR1 and COLOR2 must be supplied as hex strings with a leading #."
          (blue (/ (+ (nth 2 c1) (nth 2 c2)) 2)))
     (color-rgb-to-hex red green blue)))
 
+(defun pl/color-xyz-to-apple-rgb (X Y Z)
+  "Convert CIE X Y Z colors to Apple RGB color space."
+  (let ((r (+ (* 3.2404542 X) (* -1.5371385 Y) (* -0.4985314 Z)))
+        (g (+ (* -0.9692660 X) (* 1.8760108 Y) (* 0.0415560 Z)))
+        (b (+ (* 0.0556434 X) (* -0.2040259 Y) (* 1.0572252 Z))))
+    (list (expt r (/ 1.8)) (expt g (/ 1.8)) (expt b (/ 1.8)))))
+
+(defun pl/color-srgb-to-apple-rgb (red green blue)
+  "Convert RED GREEN BLUE colors from sRGB color space to Apple RGB.
+RED, GREEN and BLUE should be between 0.0 and 1.0, inclusive."
+  (apply 'pl/color-xyz-to-apple-rgb (color-srgb-to-xyz red green blue)))
+
 (defun pl/hex-color (color)
   "Get the hexadecimal value of COLOR."
   (when color
-    (apply 'color-rgb-to-hex (color-name-to-rgb color))))
+    (let ((srgb-color (color-name-to-rgb color)))
+      (if powerline-image-apple-rgb
+          (apply 'color-rgb-to-hex (apply 'pl/color-srgb-to-apple-rgb srgb-color))
+        (apply 'color-rgb-to-hex srgb-color)))))
 
 (defun pl/pattern (lst)
   "Turn LST into an infinite pattern."

powerline.el

@@ -112,6 +112,15 @@ This is needed to make sure that text is properly aligned."
   :group 'powerline
   :type 'boolean)
 
+(defcustom powerline-image-apple-rgb nil
+  "When t, Use Apple RGB colors for image generation.
+
+On MacOS, sRGB colors are used for all GUI elements, except for image generation
+which uses Apple RGB color space.  When this option is set, theme colors are
+converted to equivalent Apple RGB colors before image generation."
+  :group 'powerline
+  :type 'boolean)
+
 (defcustom powerline-gui-use-vcs-glyph nil
   "Display a unicode character to represent a version control system. Not always supported in GUI."
   :group 'powerline