Blender Git Loki

Reorganizing Bullet world rebuilding in scene updates to allow pure
collision-only time steps and (uncached) ghost objects.

This will allow non-rigidbody sims (e.g. hair, softbody) to hook into
bullet collision detection via "ghost objects". These are Bullet objects
without rigid body dynamics (btCollisionObject). The contacts are
generated just like for rigid bodies, but their internal solvers can
then work like before without having to use Bullet stepping.