Round save selection to nearest `pixel.size`

Hey,

I was messin with some pixels yesterday and made some awesome pictures.
When I came to saving them, I would have liked it if the selection dragger thingamy snapped to grid rather than giving me an odd sized image.

Anyhow, I've made the selection round to the nearest `pixel.size` when it generates the image.
It doesn't unfortunately show the overlay snapping to the grid, but the end result is legit.

![](http://bukk.it/noidea-jingle.gif)