Move draw_util.c to libi3.

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

i3bar/include/outputs.h

@@ -13,7 +13,6 @@
 #include <cairo/cairo-xcb.h>
 
 #include "common.h"
-#include "draw_util.h"
 
 typedef struct i3_output i3_output;