Various minor PLBVH related changes.

-Use most significant bit instead of negative for internal nodes.
-Explicitly store root node index, so that it does not have to be 0.
-Check the root node first in PLBVH traversal.
-Fix rigid body clipping in RaytracedShadowDemo.
Fix PLBVH with 0/1 nodes, fix optimized ray-AABB intersect.
Split PLBVH files into .h/.cpp.

Also move PLBVH binary tree construction into separate function.
Add only small AABBs to PLBVH, add large AABB support.