uv的反转
https://catlikecoding.com/unity/tutorials/custom-srp/post-processing/
摘抄一句话：
we finally get the original image back, but in some cases it is upside down, typically in the scene window.
this depends on the graphics API and the types of the source and destination.
this happens because some graphics APIs have the texture V coordinate start at the top while others have it start at the bottom.
fortunately unity indicates whether a manual flip is needed via the X component of a _ProjectionParams vector, which we should define…
float4 _ProjectionParams;
有两种使用方式:
1.包含头文件UnityShaderVariables.cginc
或者
2.自己手动声明。