support compound versus compound collision shape acceleration on GPU, using aabb tree versus aabb tree.
Remove constructor from b3Vector3,  to make it a POD type, so it can go into a union (and more compatible with OpenCL float4)
Use b3MakeVector3 instead of constructor
Share some code between C++ and GPU in a shared file: see b3TransformAabb2 in src/Bullet3Collision/BroadPhaseCollision/shared/b3Aabb.h
Improve PairBench a bit, show timings and #overlapping pairs.
Increase shadowmap default size to 8192x8192 (hope the GPU supports it)