Plbvh raycast2

The increase in memory consumption from splitting the m_raytracePairsKernel (8 -> 56 bytes per pair)
is rather high. Since the rigid body index is already known for each ray-rigid pair and it is possible to interpolate the ray's m_hitPoint from its m_hitFraction, we can just store the normal and put the
m_hitFraction in the normal's w component. This would reduce the memory usage to 24 bytes per pair.

Does this sound like a good idea? If so, I'll make another commit to implement it.