Fixed vertical SSAO blur

During the vertical pass, `key` didn't get a valid depth-value because of the `#ifdef`-disabled passthrough above---resulting in the pass being essentially disabled. See http://i.imgur.com/IPme7lD.png (right is with this fix).

The original SAO-paper always does the passthrough (no `#ifdef HORIZONTAL` above), but since you made the change I just updated `unpackKey` instead of removing the `#ifdef`. Doesn't matter either way I think. :)