Futher optimize `tile.png` and `tile-wide.png`

 * `tile.png`:

   before:         3498 B
   after:          3495 B
   ──────────────────────
   reduction:         3 B [~0.09%]

 * `tile-wide.png`

   before:         1866 B
   after:          1864 B
   ──────────────────────
   reduction:         2 B [~0.11%]

Close h5bp/html5-boilerplate#1604.