remove whitespace from lift gate icon

The lift gate icon had a lot of unnecessary whitespace which didn't look very nice especially on highway=track:
![liftgate_current](https://f.cloud.github.com/assets/5783139/2284600/3b9d8ef2-9fcb-11e3-9124-643207e97a04.png)

This commit removes the whitespace and adds a transparent background to the lift gate icon. 
It will look like this now:
![lift_gate_without_outline](https://f.cloud.github.com/assets/5783139/2284626/78289736-9fcb-11e3-8109-b68d1a840382.png)