General Balance and fixes

- Turns out our game ramps up fairly quickly after the first area. I've
changed a few things in the spawner but I'm still working on it.
- I forgot to mention last sync, but asteroids now explode if they're
within a slightly larger area around the planet than before to make it
easier for the centipedes to find the planet and harder to build a wall.
- centipedes that miraculously find their way off-screen are now
destroyed upon exiting the field.