implemented patch by Vroonsh to optimize island management with many static objects.

It can be disabled by commenting out the #define STATIC_SIMULATION_ISLAND_OPTIMIZATION in Bullet/src/BulletCollision/CollisionDispatch/btUnionFind.h

See also Issue 406

src/BulletCollision/CollisionDispatch/btUnionFind.cpp

@@ -70,7 +70,9 @@ void	btUnionFind::sortIslands()
 	for (int i=0;i<numElements;i++)
 	{
 		m_elements[i].m_id = find(i);
+#ifndef STATIC_SIMULATION_ISLAND_OPTIMIZATION
 		m_elements[i].m_sz = i;
+#endif //STATIC_SIMULATION_ISLAND_OPTIMIZATION
 	}
 	
 	 // Sort the vector using predicate and std::sort
@@ -78,4 +80,3 @@ void	btUnionFind::sortIslands()
 	  m_elements.quickSort(btUnionFindElementSortPredicate());
 
 }
-