Cyber Crime Driver week 3

Main changes from this week:

• there are now three different levels
• there's an overworld level as well
• briefcases spawn in pairs rather than one at a time (and in less swingy locations)

I also wrote up some new enemy AI this week, with the idea being that on one level you'd be up against the copter (which moves around randomly constantly spawning cops), while in another you'd be up against the detective (who doesn't spawn as many cops, but is actively & intelligently hunting you down), and then more different adversaries in more different levels. I actually finished implementing the detective, but it turned out to be No Fun, and I realised I wouldn't have enough time to put together any introductory material either. New players would be presented with the current copter tutorial, and then thrown into something that didn't obey those mechanics at all. 

One of the main issues with the game was (and still is) that it's very luck-dependent. A round where the briefcases consistently spawn in inconvenient positions is unwinnable, one where they are always nearby is trivial. It was also possible that the briefcase would spawn *on* you, and it being a zugzwang game you'd have to move away and then back, which absolutely sucks. So, now briefcases will spawn in pairs, and there's some code to ensure that it's very unlikely they'll  spawn right next to you (or each other). I felt it was important to still allow for lucky and unlucky spawns, just because it's nice to feel lucky, and it's also nice to have an unlucky thing that you can blame a loss on rather than yourself.

Knowing that the introductory materials are a bottleneck for introducing new mechanics, I think it's necessary that I revise them again so that different abilities/mechanics will be presented when relevant, rather than just two hardcoded pngs that appear in order when the game starts. That'll probably take up most if not all of this week's development so I expect next week's update will be pretty slim pickings.

As always, let me know what you think of the changes and/or the game generally.