Viewport window

A new feature I added, inspired from OpenTTD. You can now open multiple viewports which can independently be panned and zoomed. I think this will be even more useful when they can be independently rotated (all viewports on RCT2 currently share the same rotation at the moment) particularly when building scenery.

Opening a viewport...
![image](https://cloud.githubusercontent.com/assets/1482259/4606448/0d2dc832-521f-11e4-8b79-52661da14ed8.png)

Using viewports:
![image](https://cloud.githubusercontent.com/assets/1482259/4606463/691d45a0-521f-11e4-98f8-f02247b7c906.png)