... which calculates the average color of the pickable over a given
area.  Use this function in gimp_pickable_pick_color() when
sample_average is TRUE, and provide a default implementation which
calculates the average color using GimpPickable::get_pixel_at(), as
gimp_pickable_pick_color() did before.

The default implementation is rather slow; classes that implement
the GimpPickable interface can provide a faster specialized version
(see the next commit).