【SKNode的渲染顺序】
SKNode的zPosition属性，指定相对于其父视图的z轴坐标。
The default value is 0.0. The positive z axis is projected toward the viewer so that nodes with larger z values are closer to the viewer.
zPosition越大，则越接近用户，即zPosition值大者会把小的给覆盖掉。
When a node tree is rendered, the height of each node (in absolute coordinates) is calculated and then all nodes in the tree are rendered from smallest z value to largest z value. If multiple nodes share the same z position, those nodes are sorted so that parent nodes are drawn before their children, and siblings are rendered in the order that they appear in their parent’s children array. Hit-testing is processed in the opposite order.
当一棵树被渲染。1）会计算每个结点zPosition的绝对高度。2）按zPosition绝对值从小到大排序，然后从小到大依次渲染。3）如果2个node的zPosition相等，则优先绘制父类，再绘制子类；或按结果在children array中的次序绘制。
The SKView class’s ignoresSiblingOrder property controls whether node sorting is enabled for nodes at the same z position.
