consistency check when adding a Body to, or removing a Body from World
Equation optimization
optimized narrowphase
using array of object keys in Narrowphase.collidingBodiesLastStep. This enables optimization in V8
using garbage free Array.splice
no need to compute gravity vector length in World.step any more
added property .applyGravity to World
optimized narrowphase loop a bit
reusing array objects in .planeLine and .circleLine of Narrowphase