refactor tilemap xml format index count.

change tilemap xml mat index storage from layer->_tiles[tilesAmount - 1] to _xmlTileIndex.
The previous PR link:
https://github.com/cocos2d/cocos2d-x/pull/3420
The discussion link:
https://github.com/cocos2d/cocos2d-x/commit/bd876e13b4edf9e31e3a2d0340e51abc045e9859#commitcomment-5315247