The marker arrows in the color sliders are usually a
single color and are easy to lose track of when moving
into a range of the slider that has a similar color.
By switching from a GTK+ arrow, who's look is
determined by the GTK+ theme in action at the moment,
to custom drawn arrows, it's better to match the custom
drawn slider trough.
This patch as-is gives two small black arrows with
white borders. That's the most common solition for
things such as subtitles, karaoke, and general video
work. It also saves work not having to determine the
relative value of the part of the slider image that
it's over at the moment.
This code is easy to tweak to get a solid white and
solid black triangle (as The Gimp does), but then some
find that to be hard to 'track' as the arrow moves from
light to dark areas in a slider.
I believe that this addresses half of bug #830500