Blender Git Loki

Added time stepping in the modifier, like all the other simulations do.

This is pretty crappy in general, but there is no good alternative so
far. Eventually we'd like to separate simulation stepping from the
keyframe animation system, so that simulations can be advanced
independently. This would be very useful for tweaking (in a otherwise
constant environment), doing pre-roll, setting start frame in the cache
etc.

For now this modifier update approach is just a quick hack to get
visual feedback for the integrator.