想在碰撞后把一个shape删除，在onContactSeperate中处理会引起下面运行时错误：
Aborting due to Chipmunk error: This operation cannot be done safely during a call to cpSpaceStep() or during a query. Put these calls into a post-step callback.
Failed condition: !space->locked
所以想用旧的方式cpSpaceAddPostStepCallback来处理，但没找到cpSpace在哪可以获得。
