使得navmesh加载流程支持多层navmesh数据加载。

 DEBUG cellapp [0x00005788] [2014-05-20 17:51:29,735] - NavMeshHandle::create: (xinshoucun), layer=0
 DEBUG cellapp [0x00005788] [2014-05-20 17:51:29,766] - 	==> tiles loaded: 100
 DEBUG cellapp [0x00005788] [2014-05-20 17:51:29,766] - 	==> BVTree nodes: 30618
 DEBUG cellapp [0x00005788] [2014-05-20 17:51:29,766] - 	==> 15309 polygons (31370 vertices)
 DEBUG cellapp [0x00005788] [2014-05-20 17:51:29,766] - 	==> 40639 triangles (9569 vertices)
 DEBUG cellapp [0x00005788] [2014-05-20 17:51:29,766] - 	==> 2.46 MB of data (not including pointers)
 DEBUG cellapp [0x00005788] [2014-05-20 17:51:29,766] - NavMeshHandle::create: (xinshoucun), layer=1
 DEBUG cellapp [0x00005788] [2014-05-20 17:51:29,782] - 	==> tiles loaded: 25
 DEBUG cellapp [0x00005788] [2014-05-20 17:51:29,782] - 	==> BVTree nodes: 16758
 DEBUG cellapp [0x00005788] [2014-05-20 17:51:29,782] - 	==> 8379 polygons (15458 vertices)
 DEBUG cellapp [0x00005788] [2014-05-20 17:51:29,782] - 	==> 24058 triangles (6150 vertices)
 DEBUG cellapp [0x00005788] [2014-05-20 17:51:29,782] - 	==> 1.34 MB of data (not including pointers)