Small emergency patch for crash on tutorial.

I just put out a small emergency fix because of a crash for new people playing the first level online.

The bug was when overwriting the AI pattern for levels that have non-default behaviour. Sorry for those people that spotted the crash, it's fixed now.

There were a small number of other features I had been working on that I have deployed with this. The two main ones:

• Enemies off the screen are more likely to pathfind back onto the screen faster

• Enemy bullets can change speed in the air, this is more for aesthetics than balancing enemies.

Lots of love David Strachan