This patch fixes a bug when copying one surface to another.
Since it only exposes itself when used with non-trivial source
coordinates, it didn't surface before when only used for i3bar.
relates to #1278
In order to prepare for using cairo for rendering i3 decorations,
we need to make the draw_util.c from i3bar available via libi3 such
that both i3bar and i3 can use it.
relates to #1278