Use transferrable objects to speed up transfers between web workers

This uses [transferrable objects](http://updates.html5rocks.com/2011/12/Transferable-Objects-Lightning-Fast) to speed up buffer transfers between web workers so there are no copies.  On my machine in Chrome, this speeds the demo up by about 100ms. On the highest quality setting with no unsharp, without transferrable objects I see an average of around 450ms for the resize. With transferrable objects, I see an average of around 350ms.  The difference is similar in Firefox.