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)

src/Bullet3Common/shared/b3Int2.h

@@ -16,6 +16,8 @@ subject to the following restrictions:
 #ifndef B3_INT2_H
 #define B3_INT2_H
 
+#ifdef __cplusplus
+
 struct b3UnsignedInt2
 {
 	union
@@ -52,5 +54,11 @@ inline b3Int2 b3MakeInt2(int x, int y)
 	v.s[0] = x; v.s[1] = y;
 	return v;
 }
+#else
 
+#define b3UnsignedInt2 uint2
+#define b3Int2 int2
+#define b3MakeInt2 (int2)
+
+#endif //__cplusplus
 #endif