Improve performance and fix SDL dependency

The 'atom' SDL library went defunct recently, so I had to update to use the neagix repo.
I was hunting some major performance issues with this little program, It seemed to be fast at first but would slow down and lag response to events after a few seconds. Which at first I thought was due to the synchronization of memory in the goroutine. I removed the goroutine, no luck. So I added some simple time profiling around the core three areas: events, simulation, and graphics. The simulation was taking up 90% of the runtime. Digging deeper I realized that I created a slice of Node pointers called "active" which I appended to but never cleared. Removing the active slice shortcut and instead simply iterating over each Node showed a dramatic improvement since I'm no longer O(n) where n is iterations.