Added a few backgrounds to test the background rotation code.
The backgrouns are really ugly gradients at the moment, artists are welcome to design something better.

A waterfall, or a large cave, or rocks or dirt would be good candidates; since the background should not be too distracting, and since it is re-rendered at window resize time and at level loading time, it's better for it to not be too complex. The geek-city background does not seem slow to load on my laptop, so that may be a good reference point.