Parse colors as color_t instead of colorpixel.

With this patch we remove the temporary draw_util_colorpixel_to_color
function we introduced previously by parsing the colors as color_t to
begin with.

relates to #1278

src/resize.c

@@ -146,7 +146,7 @@ int resize_graphical_handler(Con *first, Con *second, orientation_t orientation,
     }
 
     mask = XCB_CW_BACK_PIXEL;
-    values[0] = config.client.focused.border;
+    values[0] = config.client.focused.border.colorpixel;
 
     mask |= XCB_CW_OVERRIDE_REDIRECT;
     values[1] = 1;