Blender Git Loki

Render Branch: hair collision time stepping test. Not sure this is much
better than before but it helps in some cases. It seems to me that
without this it is taking 1.5 steps each time instead of 1.

The original paper defines vi+1/2 = (xi+1 - xi)/dt, but in our case we
already have vi+1 = (xi+1 - xi)/dt ... so there's a conflict here.