bug#15837: 24.3; invalid colour conversion from Gtk+3 to X11

[Łukasz Stelmach]

Hello,

The function[1] that reads foreground and background colours from Gtk+3
(GdkRGBA) and converts them to X11 ones (XColor) uses improper
intermediate representation. The Gtk coulour is formated as
rgbi:<red>/<green>/<blue>. Although values prefixed with rgbi: are meant
to be floating point values like the ones returned by
gtk_style_context_get_*() functions but unlike Gtk colours they are
meant to be linear intensity values and will be gamma corrected[2] using
the tables[3] in Xlib.

You can check the effect for yourself by running:

xterm -bg '#777'
xterm -bg 'rgb:7777/7777/7777'
xterm -bg 'rgb:0.46667/0.46667/0.46667'

The last one will be significantly lighter.

0.46667/0.46667/0.46667 is the value returned by Gtk when a colour is
set to #777.

A patch will follow this report.

References:

[1] 
http://git.savannah.gnu.org/cgit/emacs.git/tree/src/gtkutil.c?id=c09a36ec78af479cb2ea39bf4bca8743cecd08df#n601

[2] 
http://cgit.freedesktop.org/xorg/doc/xorg-docs/tree/man/X.man?id=66e800c69fa5348b01e4b9670d6274ba392db7e5#n562

[3] 
http://cgit.freedesktop.org/xorg/lib/libX11/tree/src/xcms/LRGB.c?id=6cb02b166361200da35ba14f52cd9aaa493eb0ea#n224
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Było mi bardzo miło.                                  --- Rurku. --- ...
>Łukasz<                                --- To dobrze, że mnie słuchasz.

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