Blender Git Loki

Extended RigidBodyWorld stepping function with a tick callback.

This will allow hooking other simulations (e.g. hair) into the Bullet
time step loop to enable sync'ed stepping of Bullet and other solvers
for accurate collision response.

This means that more solvers have to become global (instead of object-
based), but that was a design goal anyway for reasons of avoiding
dependency issues.

The Bullet time step will have to be run more generally if there are
other simulations. In particular it should still be executed even when
rigid bodies are cached, because the collision info may be needed. In
case of cached rigid bodies it would probably be sufficient to make them
all kinematic bodies, so that no calculation takes place for the rigid
bodies themselves, while their collision shapes are still available and
interpolated between cache frames.