Makes transparency work for all xmobar positionings.

I think this was one of the issues with the xmobar transparency upstreaming. This just adds x and y coordinates to the pixmap so it grabs the correct chunk of the root window, so it should work for the various positions. Tested with bottom-bar and external monitor.
![cropped](https://f.cloud.github.com/assets/1454657/1142969/814d6dde-1d0b-11e3-8852-0eda3e8bf4cf.png)