Blender Git Loki

Fix for meaningless position updates during the force integration loop.

Setting the "next" state of positions inside that loop will overwrite
its own result until the last iteration. The velocities are still
integrated correctly though, only the position update was applied
incorrectly. Force calculation was also using the larger original time
step.

The fix now exposes an awkward dependency of stiffness and damping
values on the substep count. This is entirely expected, but it might
be a good idea to scale user-defined values by the inverse timestep,
so that results are comparable and can be tweaked more easily when
increasing/decreasing substeps.