Fix a couple small visual bugs

Now parallax images don't go all over the place when the player moves.
![screenshot_0](https://cloud.githubusercontent.com/assets/4390703/2764247/11094936-ca11-11e3-9c1d-f233c0d965ed.png)
The saturation filter doesn't break the transparency.
![screenshot_1](https://cloud.githubusercontent.com/assets/4390703/2764246/10e6f660-ca11-11e3-81b2-243330bd1c55.png)