Updates to environment complexity and some AI.

Although I'm probably making life more difficult for myself than is strictly necessary, I decided I wanted to do a good job on the AI of the Scourge and make them an enemy you had to outwit as well as outgun.  Each one of them counts as its own 2D dynamics object, and applies a couple of thrusters in order to navigate around the world.  Coupled with the first implementation of the navigational node system, this produces a result something like this:

The Scourge on a circular patrol route using their basic untuned P+D controller and singularly failing to navigate around some of the new flair objects.

Although this at least validates a lot of the underlying ideas, there's still a good amount of work to do.

1) Each of the individual enemies does not interact in any way with its neighbours, meaning that although there is some impression of a swarm simply due to numbers they don't really exhibit any swarm behaviour.  I have plans to upgrade this to so that they will dynamically track those in front of them, leading to much more realistic swarming.
2) At the moment the swarms stay much too bunched up, which is a combination of the fact that they have no knowledge of/collision detection with other members of the swarm, and also because they still aim squarely for the centre of navigation nodes at the moment, rather than exploiting their full radius.

Ideally, I would like an attacking swarm to be able to peel off automatically into subswarms that will attempt to attack the player from multiple angles using a variety of tactics.  For example, against weapons with a large spread it may be better to bunch up and rush the player, whereas for weapons with a single large beam scattering and attacking individually may be superior.