code: function RigidBody(_aabb, mass, friction, restitution, gravMult, onCollide, autoStep) {
      this.aabb = new aabb(_aabb.base, _aabb.vec) // clone
      this.mass = mass
      this.friction = friction
      this.restitution = restitution
      this.gravityMultiplier = gravMult
      this.onCollide = onCollide
      this.autoStep = !!autoStep
      this.airDrag = -1   // overrides global airDrag when >= 0
      this.fluidDrag = -1 // overrides global fluidDrag when >= 0
      this.onStep = null
      
      // internals
      this.velocity = vec3.create()
      this.resting = [0, 0, 0]
      this.inFluid = false
      this._ratioInFluid = 0
      this._forces = vec3.create()
      this._impulses = vec3.create()
      this._sleepFrameCount = 10 | 0
  }
docs: /*
  RIGID BODY - internal data structure
  Only AABB bodies right now. Someday will likely need spheres?