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

 * `tile.png`:

    before:        13743 B
    after:          3495 B
    ──────────────────────
    reduction:     10248 B [74.56%]

 * `tile-wide.png`

    before:         6774 B
    after:          1864 B
    ──────────────────────
    reduction:      4910 B [72.48%]

Close h5bp/html5-boilerplate#1604.