Bullet, small changes

- Add btCollisionObject#setUserValue / getUserValue
- Add btBroadphasePairArray#getCollisionObjectsValue
- Change the tests to set the index of the entity as user value of the collision object
- Update FrustumCullingTest to use these methods

My previous implementation of FrustumCullingTest updated bullet twice (once for the whole world and once for the frustums broadphase cache) and had quite an overhead because of the LongMap lookup. This change removes that overhead. Also, it now shows a more general use: getEntitiesCollidingWithObject, which is not limited to frustum culling.