Object Pool / free list – Show how particles are initialized

[munificent]

After moving the particle's state under the live union, it should also show an updated init() method where that state is set. Something like:

  void init(double x, double y,
            double xVel, double yVel, int lifetime)
  {
    live.x = x;
    live.y = y;
    live.xVel = xVel;
    live.yVel = yVel;
    framesLeft_ = lifetime;
  }