Obey the client’s border_width setting (Thanks shatter)

xterm by default sets a border_width of 2. This was not taken into
account when determining the size of the window by i3. Still, you
probably want to set this to 0 in your .Xresources as the pixels
are just lost.

include/data.h

@@ -352,6 +352,9 @@ struct Client {
         int base_height;
         int base_width;
 
+        /** The amount of pixels which X will draw around the client. */
+        int border_width;
+
         /** contains the minimum increment size as specified for the window
          * (in pixels). */
         int width_increment;