UI: Hi-res background for geoscape.

Render alternative hi-res geoscape background if base resolution is higher than default. Provided background image is fit for base resolution of 540x270 (which is perfect for display resolutions with AR 16:9).

![bghires](https://f.cloud.github.com/assets/5825656/1456177/850e837c-4326-11e3-9045-aa10874c8981.png)