如何查看shader性能消耗
1.Alt 8切换Shader Complexity mode查看shader性能消耗 (Alt 4切换回Lit Mode正常光照模式)
2.材质蓝图里查看instructions数量（如图中箭头所示）
3.直接在target platform上测试是最准确的。
（running fewer instructions, shader would perform better, but its not accurate because each instruction take different time）
汇总：Measuring shader performance
Method #1 Shader complexity View Mode
Method #2 Instruction Count
Method #3 Test on your target platform
优化材质的方法 Optimizing shader performance
Method #1 Get rid of anything you're not using
Method #2 Refactor math formulas
Method #3 'Pipelining' -Combining components into float4s to do less math
Method #4 Texture channel packing (Metalic Roughness Nornam AO)
