CSS3 defines a somewhat odd syntax for rgba() colors - the rgb values
are integers from 0 to 255 or percentages and the a value is a
float from 0 to 1. To avoid increasing the total amount of confusion
in the world, make gdk_rgb_to_string() and gdk_rgb_parse() follow
this syntax rather than using floats for r, g, and b.
https://bugzilla.gnome.org/show_bug.cgi?id=633762
GdkRGBA is a boxed struct similar to GdkColor, with the difference
that it stores alpha information as well, and colors are stored in
[0..1] doubles, in the cairo spirit.
gdk_cairo_set_source_rgba() has been also added to allow easier handling
of this new type.
