Never allow partial pixel values for scroll

I was getting partial pixels for `scrollTop`/`Left` which were making things really blurry. This does a `Math.round`. Check it:

![screen shot 2014-06-24 at 6 10 11 pm](https://cloud.githubusercontent.com/assets/69169/3380409/84bdb06c-fc05-11e3-946e-1ef0a591d747.png)

I was able to consistently repro on an external display, full height with `display-buffer.coffee` open, and doing a symbol search for `pixelPositionForScreenRow`.