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)

Demos3/GpuDemos/GpuDemo.h

@@ -48,14 +48,14 @@ public:
 	arraySizeZ(10),
 	#else
 
-					arraySizeX(30),
+		arraySizeX(30),
 		arraySizeY(30),
 		arraySizeZ(30),
 #endif
 		m_useConcaveMesh(false),
-		gapX(14.3),
-		gapY(14.0),
-		gapZ(14.3),
+		gapX(16.3),
+		gapY(6.3),
+		gapZ(16.3),
 		m_useInstancedCollisionShapes(true),
                     m_instancingRenderer(0),
 					m_window(0),